Yu-sin is an eloquent, educated, and polite man by all means, speaking slowly but passionately about his dreams and his wishes for the future. He is very well-read and in the modern era, seeks to spend whatever free time he may have reading up on modern literature and picking the minds of military tacticians before and after his life. Though he is a loyal Servant, he does not hesitate to speak his mind and will often lecture his Master if they make any misstep that he could’ve rectified.
However, in combat, this quiet man seems to become another person entirely. All of his normal formal composure vanishes, leaving a subtly smiling warrior in gold who discards all hesitation to throw himself into a battle. This is where he thrives, grinning to himself as he is locked in deathmatch after deathmatch in a Holy Grail War. Though he may be a knight, Yu-sin’s experiences on the battlefield have taught him that there is little room for honor, and he does not hesitate to fight dirty in pursuit of his goals.
His once quiet voice is raised to a bloodthirsty roar as he loses himself in bloodshed and gives in to his own anger, fighting in a frenzy that does nothing to diminish his incredible swordsmanship. While Yu-sin is a Servant that prizes clever tactics, what honor he does have bars him from refusing a challenge or fleeing from a fight. He would rather die than back down, and seeing such sentiments in his master is sure to gain them his contempt.
The same devotion Yu-sin shows in the heat of battle drives him forth in all his life, making for an extremely stubborn and persistent person who will do whatever it takes to gain an advantage. While he fights wildly in combat, outside, he more subtly works to shift situations in his favor, manipulating his opponents and allies to lead them to victorious situations. Believing that anything can be accomplished by those who try hard enough, Yu-sin is irritated by superstitions that hold others back, but is happy to use these same superstitions to manipulate people.
Due to his own strong beliefs regarding personal agency, Yu-sin regards those who back out from promises, flee from battle, and break under pressure with barely contained disgust, and will expect his master to give everything their all, just as he expects of himself. Anything less, while unlikely to make him turn against them, will certainly make him increasingly disrespectful and irritable. Yu-sin is also incredibly proud, and insulting this pride is sure to make his otherwise obscured temper flare up.
For Yu-sin, the Holy Grail is a chance to achieve his greatest goal once more - the unification of Korea. To see it broken once again brings him great grief. He cares not who rules, so long as they are just, and has little interest in the crown himself, having accepted that he is already dead and is an existence that will inevitably fade. The Holy Grail War is enough to satisfy his need for combat and mind games, and even if he dies without succeeding, the chance to battle against history’s greatest heroes will leave him pleased, but still wanting.