At the beginning of the story, Yuuichi appears to be a diligent and happy student, who works hard to be able to join his friends on the school field trip. He values his friends heavily throughout the story. He shows kindness and a willingness to self-sacrifice for the sake of his friends, even going so far as to brand himself the traitor in order to prevent further discord.
As the story progresses and the games become more and more serious, however, Yuuichi begins to show a much more sinister side. This dark nature is likely the result of his troubled upbringing by an adopted mother who had to sell her body and a con man/swindler who served as a father figure and teacher. He shows little remorse for any of his more devious actions and appears sadistic at points when he takes pleasure in the misfortunes of his enemies.
It is revealed in Chapter 99 that Yuuichi Katagiri is the person who came up with the idea of Tomodachi Game.
