While Yu-Hon was still alive, the war hero placed great pride onto his son, willingly showing him military exercises and bringing him along for 5 Tribe meetings as par for those born in a royal household. He told Yon-Hi that Soo-Won was a clever child would one day become a great man who would leave his mark upon history. He is shown to encourage Soo-Won to the point of telling him to beat a simply sickness to death. Due to his strength and intelligence, Soo-Won looked up to him and dreamed that he could fight by his side and die for him on the battlefield someday. This dream of Soo-Won was shattered when Yu-Hon was killed by King Il by stabbing and being pushed off a cliff, when Soo-Won was just 9 years old. During the aftermath of Yu-Hon's death, Soo-Won exhumed his father's grave and performed an autopsy, and confirmed that he had been stabbed by King Il.
The death of Yu-Hon extremely traumatized and devastated Soo-Won, eventually becoming the root cause of his mission to avenge him and betray Yona and Hak by usurping the throne. In Soo-Won's words, he has inherited Yu-Hon's dying wish and as his son vows to restore Kouka into a powerful nation.
Soo-Won regards his father with great pride, respect, and affection, and his father is arguably a great part of influences him to take action. When his mother is shaken by Yu-Hon's death, Soo-Won vows to become Yu-Hon's "replacement" in order to care for her.
However, Soo-Won's admiration for his father has shown signs of been slightly misguided, as shown during the Xing arc when he justifies Yu-Hon's barbaric acts as morally correct against against Xing, that brutally scarred the first crown Princess of Xing, Kouren, who began harboring a great hatred for Yu-Hon and indirectly Soo-Won himself. When Soo-Won was on his way to war against Xing he was confronted by Yona, who called him out on his father's acts and implied that Yu-Hon's unnecessary cruelty is what caused him to lose the throne to Il.
Yong-Hi was Soo-Won‘s mother. As he takes after his mother's looks, he also took after her bloodline as a descendant of Hiryuu. To Yong-Hi, Soo-Won was a talkative, curious, intelligent and mischievous child - she likened him as a "blessing" and noted that as a child, be it language or physical prowess, he was 3 times as talented as normal people. Soo-Won deeply cared for his mother, going as far as
While living in Hiryuu Castle, Soo-Won often ended up leaving his room and going on adventures, leading Yong-Hi to feel frustrated and compare him to "a missing cat". Often worried about his safety, she would watch over him playing with Yona and Hak in the castle gardens. When she became sick with the Crimson illness, Soo-Won had to leave Hiryuu Castle with his parents and move to a separate villa residence so she could get better.
Unfortunately, Yong-Hi knew that there was no cure to the Crimson illness, and prayed that her son would not go through her pain. Later becoming bedbound as she got sicker and sicker, and shaken in the aftermath of Kashi and Yu-Hon's deaths, Soo-Won put up a strong front. Deeply caring for his mother, Soo-Won went as as far as to promise to stay with his mother and take on the responsibilities Yu-Hon had once taken for her welfare.
Upon discovering Yu-Hon had been killed by Il, Soo-Won told his mother that he would investigate into Il's motives and then decide whether or not he will kill him.
After realizing that her son was showing bloodlust towards Il for killing Yu-Hon, no longer mentioned Il in conversation, and now continued to invite Yu-Hon's political supporters in and out of their mansion, Yong-Hi became frightened about Soo-Won's sudden change in behavior and wrote to King Il about her fears that her son would cause a calamity upon Il and Yona in the case that King Il did not rule well. Begging King Il to rule well and resolve the situation, Yong-Hi voiced her wish in her memoir that her son and King Il would not hate one another, and that he and Yona would not start hating one another in order to break the cycle of death. Shortly after writing this wish, Yong-Hi died, presumably leaving Soo-Won behind with no parents or immediate family.
Around 10 years later, when Soo-Won began suffering from an attack of the Crimson Illness, he falls unconscious from the pain, and upon hearing someone telling him to calm down, wonders if it is his mother who has come to comfort him, noting that the pain has lessened a little.
When getting ready to conduct a conference with Kai Empire diplomats, Soo-Won's health considerably worsens, and he wonders how his mother was able to stand the head pain, realizing that she was only able to survive for 3 years after getting ill.
As the only other living member of the current royal family, Yona was Soo-Won's close childhood friend before his betrayal. During their early years, they both lived and played together in Hiryuu Castle with their respective families.
Once Soo-Won's mother became sick with the Crimson Illness, Soo-Won had to move with his parents to a separate residence, and promised to visit Yona at Hiryuu Castle from time to time. From then on, Soo-Won often visited the castle with his father during 5 Tribe meetings to play with Yona and Hak. Most notably, he is the one who comforted a depressed Yona struggling from insomnia in the wake of her mother Kashi's death. Because of his kind and gentle disposition towards her, Yona fell in love with him, but he on the other hand loved her as a younger sister to be protected, oblivious to her feelings until 10 years later.
Between his father's death and eventual usurpation of the throne, Soo-Won pleaded that Hak protect Yona at all costs, knowing the 3 of them would split after he killed Emperor Il. After Yona's confession, Soo-Won began to view Yona a bit more like a woman, acknowledging that he would be too nervous to hold hands and sleep with her as they had done as children.
On Yona's 16th birthday, Soo-Won gifted Yona an exquisite flower hairpin and complimented her hair, sending her in a spiral of happiness. But the same night, Yona accidentally discovered Soo-Won killing her father in Emperor Il's room. Heartbroken and traumatized by his betrayal, Yona asked Soo-Won why he would do such a thing, and Soo-Won revealed that he had been planned to avenge his father after Emperor had killed him 10 years ago, and that he would inherit his father's will and become the next monarch of Kouka. When he asks Yona why she would walk into the king's room, noting that she rarely did so, Yona revealed that she had planned to ask her father for permission to marry Soo-Won on the premise that she could never forget him, much to his shock and guilt.
Even after betraying Yona and their friendship, he still worries for her safety. When Kan Tae-Jun informs him of her supposed "death" by a cliff fall in the Northern Mountains, he shows a grief-stricken expression clutching her hair.
About 3 months after his betrayal, while on a reconnaissance mission to Port Awa to survey the aftermath of the battles between the Awa Pirates and Yang Kum-Ji, Soo-Won unexpectedly bumped into Yona on a deserted Awa street. Among many questions on how she had lived and why she was there, Soo-Won was relieved that she had lived and assumed Hak was protecting her. When his men came running to him, Soo-Won protected Yona's identity from his guards by wrapping her in his cloak, leaving the men to misinterpret he was flirting with a local woman, to Soo-Won's chagrin. When Yona, still hidden, saw Soo-Won carrying a sword sheath, she reached for his sword, but Soo-Won stopped her. When his guards left, Soo-Won concluded that Yona in all likelihood wished to kill him, but informed her that he could not die yet, as he had a mission to fulfill. Gently touching her face with a sad smile and bidding her farewell, he left. Upon leaving the city, when his guards commented it rare for Soo-Won to step out with a woman and wondered if she was that great, Soo-Won sadly agreed and deemed Yona a "very unforgettable person", creating the question within the fandom about whether he has developed feelings for Yona.
During the Fire Tribe arc, while thwarting Kan Soo-Jin's attempt to usurp the throne on the pretext that the Fire Tribe citizens were King Hiryuu's descendants, Soo-Won glimpsed Yona and Hak on the battlefield and exchanged a stunned eye-contact with Yona, but did not ask the soldiers to capture them, leaving them alone. After meeting Tae-Jun helping impoverished Fire Tribe villagers, who accidentally slipped information about Yona in front of Soo-Won, he realized Yona was a step ahead of him as the one who provided Fire Tribe villagers with Iza seeds, and looked somberly at the sky.
During the Water Tribe arc, while on a reconnaissance mission to fix the Nadai drug trafficking problem and cooperating with General Joon-Gi's daughter An Lili, Soo-Won unexpectedly reunited with Yona and her group. Despite the initial shock and awkwardness due to their statuses as enemies and hiding their identities, Soo-Won still defended Yona from flying arrows targeting her as the enemy of drug trafficker Hiyou, and stopped her from firing arrows when learning she had suffered a back injury, out of worry for her safety. He was also shocked to see that she had taken up weapons, and when she saved him from flying arrows in return while fighting Hiyou's subordinates.
Currently, Soo-Won's relationship with Yona is extremely complicated. Due to his betrayal, Yona was left heartbroken and traumatized, and had to flee for her life with Hak. Initially she kept the hairpin Soo-Won gave her as a reminder of the night of her 16th birthday, but as time went on, she began to heal from the heartbreak and fall in love with Hak, eventually letting go of the hairpin during the Xing arc and selling it to the info breaker Ogi in exchange for information.
Yona confided with Lili on her past romantic feelings for Soo-Won, realizing that even if she never forgave Soo-Won for killing her father, she never truly wanted to kill him, because she couldn't believe that his friendship and kindness towards her and Hak was a lie. As of now, all Yona wants to understand is Soo-Won's motives and goals, realizing that he is a good influence on Kouka and can make it a better place for its citizens.
During the Xing arc, when tensions were imminent between Kouka and Xing Kingdoms, Yona later travelled to stop his troops and confront him to negotiate that he find a peaceful way to resolve the animosity between Kouka and Xing. However, he declined, knowing the first princess of Xing Kouren's hatred of Kouka.
This led to Yona questioning the morality of his father's war crimes in Xing, decapitating the heads of war prisoners he promised to release and lobbing the off the walls of castle gates as a show of power. Soo-Won excused his father's actions, stating that it often happened in war. This answer shocked Yona, and in response, she coolly pointed to his father's cruel methods as the reason Yu-Hon was not chosen to be the next king, implying that Soo-Won was not worthy of being king if he was willing to overlook his father's crimes against humanity and justified them, shocking Soo-Won and reminding him of a flashback where King Il told him Yona was the reincarnation of Hiryuu.
After returning to Hiryuu Castle thanks to Kye-Sook suggesting an alliance between Soo-Won's faction and the Dragons to avoid public insurrection, Yona later discovered by accident that Soo-Won is terminally ill with the disease inherited by his mother, Yon-Hi, and that through her lineage he is a blood descendant of Hiryuu. During military excersises with Sky Tribe soldiers, he acquired legitimate proof from the Dragons that Yona is the reincarnation of his ancestor Hiryuu, not through blood but a resonating soul, leading him to conclude her the most needless existence in the world. As of now, the future of their relationship and alliance is unclear.
From the time they were toddlers and until Soo-Won's betrayal, Hak was Soo-Won's best friend. Soo-Won first met Hak during a 5 Tribe meeting while playing in the castle gardens under the watch of his mother. When asked how they met, Soo-Won narrated that Hak had visited the castle with his adopted grandfather, Mundok, and Soo-Won had invited him to play. They climbed trees together and Soo-Won was instantly amazed by how Hak swiftly lept from branch to branch, and excited to meet a friend his age. Soo-Won also sought to introduce his new friend to Yona, and the friendship between the three friends began.
From a young age, Soo-Won loved Hak as an elder brother, and viewed him as an admirable and dependable person. The two trained in archery and spearmanship under the tutelage of Mundok, and raised a falcon named Gulfan together. 
Showing great pride in his martial arts abilities, Soo-Won attended martial arts competitions and eagerly anticipated his matches, and most notably complimented Hak right after they saved Yona from a kidnapping by the Kai Empire together with the help of Ogi and his info brokers, revealing that he had made Hak one of his goals in life to aim towards, deeply touching Hak and pushing him to live up to Soo-Won's admiration, and later become his righthand man.
However, after Yu-Hon's death, Soo-Won became more secretive and plotted to usurp the throne from King Il. A few years before the betrayal, Hak pledged to one day become Soo-Won's righthand man for life if Soo-Won married Yona and became the next king, causing Soo-Won to cry at the impossible prospect of having Hak by his side. He pleaded with Hak to protect Yona at all costs, knowing that the three of them would separate after he committed his revenge to King Il.
On Yona's 16th birthday, Soo-Won drops all honorifics with Hak and asks that he speak with him informally, but Hak does not take the hint and addresses him with the respect towards a superior, disappointing Soo-Won.
Soo-Won's betrayal would greatly shock and hurt Hak over the course of the series, as Hak learned Soo-Won killed King Il and was even willing to drive Yona out of the castle fleeing for her life. However, Soo-Won showed to still have great faith in Hak, deeming to a doubtful Kan Soo-Jin that Hak was not an idiot and would lie low to protect Yona's life.
Upon hearing Hak died in the Northern Mountains, Soo-Won is stunned and shows a grief-stricken expression. When Soo-Won learns Yona is alive in Awa, he asks where Hak is, then takes it back a stupid question, as surely Yona is alive because Hak is protecting her.
During the Fire Tribe arc, Hak spots Soo-Won on the battlefield, and the hawk they raised together, Gulfan, lands upon Soo-Won's shoulder. Hak is instantly hit with hatred and rage, but retreats on Yona's orders.
Upon their first one on one encounter after so long in the Water Tribe arc in Sensui, Hak is filled with rage and grief, to the point of crying. Just as Hak demolishes his guards and attempts to kill him, Soo-Won is rooted to the spot in shock, unable to unsheath his sword. Luckily, Jae-ha, Kija, and Yona stop Hak's fit of rage, and Soo-Won and his guards flee the scene.
Later, Joo-doh harshly criticizes Soo-Won for being unable to draw his sword and kill Hak, being swayed by his feelings and prioritizing him over Kouka. Soo-Won apologizes and sadly agrees to kill Hak the next time he gets in his way.
During the Sei arc, Hak and Soo-Won ally in a joint effort to save Lili, and are able to communicate by just exchanging glances, leaving both stunned, and Yona watching their cooperation and crying from a distance on Jae-ha, wondering why the deep friendship between Hak and Soo-Won had dissolved into enmity. When Joo-doh insists that Soo-Won kill Hak on the threat that Hak poses to his life, Soo-Won convinces Joo-doh that killing Hak will be a politically unwise move as it would dissolve the 5 Tribes.
During the Xing arc, Hak accompanied Yona representing Xing in the peace talks with Kouka, represented by Soo-Won and his troops. After negotiating with Princess Kouren and gaining Xing's military forces under Kouka as a vassal nation, Soo-Won and Hak spot each other, with Hak glaring at Soo-Won with mistrust and hatred, and Soo-Won looking at him, emotionless.
Currently, Hak and Soo-Won's relationship is extremely complicated. Hak harbors a deep grudge, hatred, and sadness at the betrayal of his best friend and effects it had on Yona, while Soo-Won shoulders much guilt.
After allying with the Dragons thanks to Kye-Sook in order to avoid public insurrection, Hak has taken it upon himself to join the Sky Tribe army and attain information about Soo-Won's motives and his reign. Hak reunited with Gulfan a long time, and when Hak asked Joo-Doh why his best friend decided to betray King Il and Yona, Joo-doh revealed to Hak that Soo-Won killed King Il because King Il killed his father, shocking Hak. While in disbelief, Hak began to wonder if it was true, and if Soo-Won had mulled over why King Il had killed his father for years on end, realizing the perspective of Soo-Won was very different from what he had thought it was.
While spotting Hak with Gulfan and the Sky Tribe soldiers on Hiryuu Castle grounds sometime later, Soo-Won was emotionally struck and later, while suffering an attack of the Crimson Illness, confessed to Yona his longing for the past days seeing Hak and Gulfan flying around so happily.
Lee Geun-Tae is the general of the Earth clan. Soo-Won goes to the Earth clan in order to help their impoverished land and to show off Guen-Tae as an admirable leader, who could boost morale very quickly. At first, Guen-Tae compares Soo-Won to Hak, saying that Hak is a better fighter, comparing Hak to a tiger. Although he compares Hak to a tiger, he compares Soo-Won to a rabbit for his skinny arms and to the fact that he has never seen Soo-Won fight. In the end, Guen-Tae has seen Soo-Won's strength and follows him as a leader and acknowledging him as king.
When Soo-won and Lili first meet, he saves Lili from a Nadai addict and ends up becoming her "bodyguard". Later, he aids her (along with Yona and her crew) defeat a fleet of drug-trading ships from Sei. After Lili is kidnapped and brought to Sei as a slave, Soo-Won sets out looking for her, along with four of the five Koukan Generals. When she is nearly hanged for attempting to escape, he ends up working very briefly with Hak to cut her from the gallows. After Lili heals, Soo-Won comes to visit her and tell her about the effect her rebellion in Sei caused, assuring her efforts had a great impact and weren't useless. Lili cries, touched by his words, and as he leaves, she muses to herself that she'd like to get to know him better for Yona's sake.
Later, she follows Suwon to Ogi's hideout and catches him exchanging money for intel on the neighbouring kingdom of Xing. Ogi reveals that rumours are floating around Xing of Yona's group, and having gotten the information he came there for, Suwon leaves along with Lili and her guards. Once outside, Lili confronts Suwon about the rumours, asking if he plans to pursue her. She goes on to say that Yona is a precious friend of hers, and that she believes Yona is important to Suwon deep down as well. Suwon immediately silences her and warns her not to mention Yona's name again.
Ever since Soo-Won was a kid, he has been keeping an eye on him and mostly became his guardian even after Yu-Hon’s death. As crucial as he is, he doesn’t let Soo-Won to go to places easily like he usually does and always reminds him that he shouldn’t be playing around as he is now the king. Despite all of his nagging, he remains loyal towards him, even after he overthrew Emperor Il from his position.
Kye-Sook is Soo-Won's advisor.
Gulfan is a falcon that was raised by Soo-Won and Hak.