Destiny
Yuri is a man who can be guided by his instincts. This can be a strength and a weakness. It means he never stops acting and doesn't hesitate. He doesn't take a time out to whine about his problems or feel sorry for himself, he always pushes ahead to the next thing. But it also leaves him at a loss and feeling listless if he isn't careful.
The beginning of the first game finds him this way. He is a grouchy, afraid, and terribly lonesome person who wanders aimlessly and saves people based on orders issued to him by a voice in his head. He only begins to find fulfillment after he meets Alice and protects her along their journey. Koudelka's own (garbled and vague) instructions that lead Yuri in that direction.
Dark... and Ligh... reun..ed... Prote... a... find yo... reas... fo... living...
What becomes Yuri's "reason for living" is never explicitly stated. I like to believe he finds it in the simple joy of living. After attempting to fuse with the Seraphic Radiance and coming so close to death, he decides that as long as he has his life, as long as he can fight, he will never give in. I think it is standing by Alice's side and becoming so close to his companions that gives him that strength. Before his journey, Yuri was alone and had no memories but those of a murdered mother and a vanished father. He could only cling to things that brought him fear and shame. He had no reason to live a life like that. This all changed when he began to live with his companions and learn about them and himself.
In Covenant, Yuri's faith in himself rises and falls. He leaves to find a cure for the Holy Mistletoe curse and sees more of the world than he had before. He finds himself wondering, watching the priests of Goreme Valley, why people suffer. Though he can go through the motions of fighting, his doubts finally stop him in his tracks as he confronts Foreign Affairs Minister Ishimura. What Yuri has been asking all along is why did Domremy have to be attacked? Why did he have to be cursed? Why did Alice have to die?
He never finds his answers because there are none. What he finds instead is his reason for fighting. Roger Bacon says it is Alice's memory, the life she gave him, and the world she loved so much. He tells Kato it is for the future he makes for himself. I believe it was a combination of all these things. He fought to live and so that the world he and Alice love would live. His philosophy is that life is hard, kind of mean, and occasionally just cruel. But it is worth living and dying for, so that is what Yuri will do.
