This article contains details from the manga that are not covered in the anime. Proceed with caution.
Soo-Won (スウォン Suwon?) is the current king of Kouka Kingdom, and the 11th king of the Sky Tribe as well as the anti-hero of the manga and anime series, Akatsuki no Yona. He is the son of the deceased General Yu-Hon and Lady Yong-Hi, the older cousin of Princess Yona, and the childhood friend of Hak. He murders his uncle, Emperor Il, to avenge the alleged murder of his father. Currently, he is resolving the internal conflicts of Kouka in order to bring the country back to the strong nation it used to be during the reign of his grandfather Emperor Joo-Nam.
Soo-Won is a tall young man with soft, delicate features. In looks, he takes after his mother Yong-Hi, while his height takes after his father, Yu-Hon. He has sea green-blue eyes and fair skin. His dark blond hair extends slightly past his shoulders and is usually tied into a side ponytail using a gold ornament (initially a small ribbon in the manga). His bangs are parted down the centre, and some strands are longer and wavier than the normal cut.
As a child, his hair worn loose and shoulder-length. He was considered by Yona to be "cute like a girl", but as he grew up his features became more masculine, while retaining some of the same softness. He is considered very handsome by his people after ascending the throne.
In the anime, he dons a simple white robe held by a purple sash under a mauve overcoat with purple hemlines. Throughout the manga, Soo-Won is shown to wear high-class robes marking the nobility, including a yellow haori worn by Kings.
Soo-Won is a complicated character. He exhibits a calm, cheerful demeanor most of the time. He has immense emotional strength and maturity, even able to stay composed surrounding the conditions around his father's death at 9, unlike other adults in the same room. He can adapt to any situation. Towards those he likes, he is kind and loving. However, Soo-Won is a very calculating individual. He has a keen observation, which helps him to see through people’s real intention. He will pretend to be naive and carefree if it serves his purposes. This is a genuine part of his personality, but also a tactic he employs to make his enemies underestimate him and divert their attention. He does not easily trust anyone, believing no one in the world is unselfish.
From a young age, he has extremely sociable and curious. However, despite all his socializing, he tends to be secretive regarding sensitive topics like his private life and past. He is not the sort who attempts to live self-sufficiently; however, once he gets into something, he is very proactive about it. In addition, he has a habit of changing or dodging a topic if someone catches him in a lie or questions his intentions. He is a bit reticent, not very expressive and sensitive.
In their youth, Hak commented that Soo-Won is interested in all people and all things equally. Soo-Won has the ability to approach anyone and never judges anyone for their appearance or social status. Thanks to this, he establishes connections with various people at a young age, even those who appear rough and uncivilized. For this, Hak realizes Soo-Won's potential to become a great King one day and admires him to the point of working to become his right-hand man.
Due to his uncanny charisma and friendliness, Soo-Won easily forms connections, but he is clueless about romantic love. Yona once commented that Soo-Won is like a child in such matters.  As he tells Lili, he has little to no understanding to how it works. Not only did he fail to notice Yona's feelings for him, but when he did notice after a decade, he second-guessed it. Growing up, he attended many marriage talks with various women, but showed little interest in pursuing any relationship. This trait of his was so infamous throughout the castle that it even caused some of the castle people to wonder if their king had a different sexuality. As king, he does not really think about finding a consort, and as Lili noted, the only point he sees to it is if there is political benefit. However, he surprisingly has some understanding to how how adult interactions between men and women work, as seen where he states to Yona that he can no longer sleep with her like they did as children.
With his frail looking build and lack of appearance in combat, he is viewed by some as weak and is often contrasted with his childhood friend Hak, a martial arts prodigy. In truth however, he never shows his real strength unless he needs to. When he does display it, he is seen to be a swordsman even capable of threatening General Lee Geun-Tae. Above all, he is an excellent tactician and a terrific strategist. He can produce battle strategies that will lead his troops efficiently and manipulate his enemies effortlessly.
Soo-Won is very lonely ,apathetic, devastated. he can be a sarcastic sometimes. He is highly determined and ambitious, such that he will even challenge the will of the Heavens if need be. As his father, Yu-Hon, disliked anything to do with the divine, Soo-Won too rejects divine will and makes it a point throughout the series that he is only interested in using "the power of the people". He has a profound love and pride for Kouka and has made it his goal to restore it into a great nation and protect it from foreign invaders, especially the imposing Kai Empire.
He also holds great admiration for strong people. He idolizes his father Yu-Hon, who was a powerful but bloodthirsty warrior, and aspires to be as strong as him. Soo-Won admired his father so much that he was willing to overlook many atrocities Yu-Hon committed when he was alive, such as a controversial incident with Xing. After his father died, Soo-Won was devastated. He became an increasingly secretive person as he planned his revenge. For this reason, he was willing to tear himself away from Hak and Yona and destroy his relationships with them to pursue his ambitions. Despite his heartless actions towards them, he still genuinely cares for his childhood friends, as was made evident when he learned of Yona’s apparent death or how he was rendered unable to unsheathe his sword after seeing the anger and pain with which Hak attacked him.
Soo-Won was born in Hiryuu Castle to General Yu-Hon and Lady Yong-Hi, and resided there for around the first 8 or so years of his life. After his mother had to move to a separate residence for medical treatment, he joined his father and visited Hiryuu Castle during 5 Tribe meetings, often spending his time playing with his close childhood friends Yona and Hak. His relationships with his friends made him happy and he wished that they would stay friends forever. However, achieving this dream became impossible when he was forced to choose between his friends and pursuing revenge and usurping the throne.
When he was nine, his father was murdered by Emperor Il while they were on a long ride on horses together, though the murder was covered up as an accident. Since then, Soo-Won harbored a secret hatred for the emperor and plotted revenge in order to become the king and fulfill his father’s dying wish to restore Kouka into a global superpower. Knowing this plot would eventually tear him, Yona, and Hak apart, he remained silent about his plans and continued on as if nothing had changed. At some point, Emperor Il told him that Yona was the reincarnation of King Hiryuu and that he (Soo-Won) was different. Later on, is revealed that King Il had always been hostile towards Soo-Won, even when he was a small child. When Yona was just born, Soo-Won went to greet her as a toddler, but King Il would not let him get close and chastised him as unpleasant. Another time, when Soo-Won and Yona were playing and about to enter Hiryuu Mausoleum together, King Il said that the mausoleum was no place for him. The night Soo-Won avenges his father, King Il tells him he will never be the Crimson Dragon King and that Yona, the rightful heir, would bring about his demise.
As the King of Kouka, Soo-Won has the power to rule over the people and land. He has full control of the kingdom. Soo-Won is able to excellently wield a sword and other weapons, from his swordmaster and later bodyguard, Hyu-Ri. Son Mundok taught him archery and spearmanship at a young age, along with Hak. This is confirmed in the anime when he shows surprising ability in the mock battle against Geun-Tae, the Earth Tribe general known to be one of the most powerful generals. Soo-won also demonstrates great political competence when dealing with the five generals as well as skill in strategy as seen in the mock battle.
Soo-Won is a highly intelligent, clever and cunning individual; these traits made him one of the best - if not the best - strategist shown so far in the series. One example of his ability was dress his soldiers' horses like tigers to scare the enemies this tactic gave his men time enough to attack before the opponent could even react. His intelligence and charisma, as well as his skills as a warrior were enough to gather the support of four of the Five Tribes (with the Wind Tribe remaining as the only exception due to their loyalty to Hak) and consolidate his authority as the ruler of the kingdom. Politics seem to be his preferred ground, and he showed amazing skills in dealing with people; that does not mean, however, that he is not a great warrior himself. Although shown in the beginning of the series that he is not as strong as Hak, he has shown be be at least near his power. As Hak has the martial ability to rival a dragon warrior, even challenging him is a considerable accomplishment.
- (To the citizens of Hiryuu Castle) "It does not matter to me if there is someone watching us from the Heavens. Because what I want is not the power of Gods, but the power of the people. I will rebuild the Kouka Kingdom into the powerful kingdom it was when the ninth king was the ruler. If anything, even a god, gets in the way, I will bring it to the ground!" 
- (To Yona) "But I love your hair, Yona. It's a beautiful crimson... just like the blush of dawn." 
- (To Hak) "Quit talking so politely. Call me Soo-Won, like you used to." 
- (To Hak) "It's because you were there, Hak. You're always so strong, Hak. Dependable... and cool. I wanted to be like that. After seeing you, I wanted to get closer to you. I wanted to become more like you. I always think that. It's frustrating. Even though we're so close... I don't feel like I can reach you at all. Everything about you... is my goal." 
- (To Yona) "King Il will never open his eyes. I killed him." 
- (To Yona) "You don't know. Yes, I loved King Il too. Even though people called him a coward for fearing conflict, I used to think that was his kindness. But I was wrong. I was so wrong." 
- [source needed] "Kindness is not a crime. But sometimes it awakens the weakness and greed in others." 
- (To Yona) "I thought that one day, I would stand alongside my father on the battlefield, and give my life for my father's sake. But after King Il succeeded the crown, he killed Yu-Hon, his own brother. Do you understand? King Il, who supposedly hated weapons and evaded strife, killed my father with a sword. Princess Yona, that is why I have lived for this day, for 10 years.To avenge my father's death, and to carry on his will, I will become Kouka's King!" 
- (To Yona and Hak) "I regret it. I wanted the three of us to rest together like we did when we had our colds... forever and ever." 
- (To Kye-Sook about Hak) "Sir Kye-Sook, do you know of the Thunder Beast from the Wind Tribe? I believe he may become the strongest solider of 100... no, 1000 years. Though I can't hope to keep him as an ally." 
- (To Hak) "I sent King Il to Hell moments ago." 
- (To the guards about Hak) "Stay back. Your heads will fly off if you get any closer. Standing before you is the trump card of Hiryuu Castle, one of the five Generals... Son Hak." 
- (To Hak) "This nation... has no need for a weak king." 
- (To Hak about the future) "Yes... that is a fortunate dream. Hak, please... protect Yona... okay?" 
- (To Hak) "The Soo-Won you knew never existed. I will strike down anyone who gets in my way. Anyone." 
- (To Kan Soo-Jin about Hak) "Hak is no idiot. If he spreads the truth and causes commotion, Princess Yona's life will be in jeopardy. To protect the princess, he is most likely lying low for now." 
- (To Son Mundok) "Thank you... I'm glad that you thought of me as a grandson, Master Mundok." 
- [source needed] "The Crimson Dragon Castle is a symbol of the king of Kouka Kingdom. I swore to myself that I would return to this castle one day." 
- [source needed] "When I become King, Yona and Hak will not be at my side. Warm days like today make me doubt myself a bit. But now my right arm is gone. And so is the girl who gave me warmth... I have made it this far by casting them off and crushing them underfoot. So I will doubt myself no longer!" 
- (To Yona about Hak) "You're safe and sound. That must be because even now, he's risking his life to the end to protect you." 
- (To Yona) "Do you wish to kill me? It's only natural that you do. But I cannot die right now, because there is something I must do. Farewell... Yona." 
- (To Yona about Hak) "He's there, isn't he? The Thunder Beast. Then, whatever comes at us, we will not lose." 
- (To Kija) "It is not my desire to sit upon the throne. I have but one goal: to make this country strong. Enough so that it won't be invaded. I only became King only as a means towards this purpose. Your existence is intriguing, and I won't refute your claim, but it is also true that I have no need for you. Because what I want is not the power of Gods, but the power of humans." 
- (To Lord Kushibi) "We, with a 'mere handful of troops,' triumphed over a thousand soldiers" 
- [source needed] "I cannot stop now. I must keep this kingdom alive." 
- He ranked 9th in the character popularity poll conducted by Hana to Yume in December, 2014. He scored 389 votes in total.
- Soo-Won is the 11th person from the Sky Tribe to be heralded as the king.
- He and Kusanagi Mizuho share the same birthday.
- Soo-Won looks very much like King Hiryuu, who is later revealed in the manga to be his ancestor. Soo-Won is the most intelligent and knowledgeable character in Akatsuki no Yona.
- Soo-Won has an odd fascination with Zeno and his dragon abilities.
- Akatsuki no Yona Chapter 43, Page 7
- Akatsuki no Yona Volume 17
- Akatsuki no Yona Fanbook
- Akatsuki no Yona Chapter 189
- Akatsuki no Yona Chapter 02, Page 13
- Akatsuki no Yona Chapter 61, Page 9
- Akatsuki no Yona Chapter 01, Page 48
- Akatsuki no Yona Chapter 45, Page 11
- Akatsuki no Yona Chapter 02, Page 11
- Akatsuki no Yona Chapter 10, Pages 28-29
- Akatsuki no Yona Chapter 92, Page 27
- Akatsuki no Yona Chapter 61, Page 3
- Akatsuki no Yona Chapter 01, Page 12
- Akatsuki no Yona Chapter 11, Pages 18-19
- Akatsuki no Yona Episode 09
- Akatsuki no Yona Chapter 39, Page 8
- Akatsuki no Yona Chapter 39, Page 17-19
- Hana to Yume (花とゆめ), 2015/1 edition
- Akatsuki no Yona Chapter 11, Page 14
- Kusanagi Mizuho - Anime News Network profile