Gods... and Buddha... Please... at least spare my little brother. He's not like me. His heart is kind. He wants... to help people but I stopped him. I'm the one who's bad. If you must punish someone... punish me... please. The truth is I knew. The "Mu" in "Muichiro" is the "Mu" in "Mugen". Infinity. For the good of others... you... can tap into infinite strength.
Yuichiro was a short young boy with long, straight hair of a dark black color that fades to pale turquoise the closer to his waist it goes. He wore it loose with two clumps protruding slightly from each side of his head, a set of marginally shorter hair left down to frame his face, and unruly bangs over his forehead. He also possessed large, mint-green eyes, a trait, like his hair, that he shared with his twin brother, Muichiro Tokito.
He wore a dark, sleeveless yukata with a white mist design, similar to the lighter one his brother wore when they were both 10.
Yuichiro was a cold and skeptical individual who didn't believe in compassion or caring for others. He believed that doing good deeds wouldn't do a person a shred of good, which is seen when he claimed his father was a fool for dying while trying to gather herbs to help cure his mother's fatal illness, and shot down Muichiro's suggestion that they become Demon Slayers in order to save people's lives. This behavior likely originated from the pain and grief of losing both of his parents in one fell swoop, however, since just before he passed away from a Demon attack, Yuichiro was seen praying that his brother be saved and apologizing for getting in his way when he wanted to be useful, suggesting that deep down he was actually a kind and caring person.
Later, upon Muichiro's own death and their reunion in the afterlife, Yuichiro is shown angrily and tearfully telling his brother to return back to the land of the living, lashing out at him for putting his life in danger and as a consequence dying at such a young age. He showed Muichiro that his rough, blunt and sometimes cruel personality was due to him trying to protect and look after his younger brother in his own way since he truly loved him.
Yuichiro and his younger twin brother, Muichiro, were born to a woodcutter and his wife. When they were 10, their mother caught a cold which developed into bronchitis and killed her. Their father, away at the time, went to fetch medicinal herbs during a storm and ended up falling off a cliff to his death.
The two siblings survived together, while Muichiro took after the emotive and kind nature of their father, Yuichiro instead held a cold and impassive outlook on life.
In the following spring, Kagaya Ubuyashiki's wife, Amane Ubuyashiki, arrived to recruit them for the Demon Slayer Corps. Muichiro's excitement was opposed by Yuichiro's unwelcoming and rejecting reception. In an outburst, Yuichiro revealed that he blamed the death of their parents on their hardworking and selfless nature and adopted a cold outlook, hoping to keep himself and his brother alive.
While Amane continued to visit the twins, Yuichiro chased her away each time. In the following summer, a Demon arrived at their cabin during the night and attacked, and as a result Yuichiro lost his left arm while trying to protect Muichiro. As the Demon badgered them for their worthlessness, Muichiro lost himself in a rage and destroyed the demon with all the tools at their disposal and wooden logs.
By sunrise, the Demon withered into ash and Muichiro dragged himself back to the house. There, he found Yuichiro near death praying to the heavens that his brother might survive. Yuichiro then told Muichiro that the 'Mu' in his name means infinity, and told him to live on. He then laid there grasping his brother's hand as he died.
- (To Muichiro Tokito) "What could you do?! You can't even boil rice by yourself! But you're gonna be a swordsman and help people?! Quit talking nonsense! You really are exactly like father and mother! You're a dreamer! What's wrong with your head?! Like mother, who didn't say she was sick and kept on working until it ruined her health! And father who went out to gather herbs in a storm despite how much I tried to stop him! Despite all the times I told mother to rest! Helping people is something only certain people can do! We're just kids! Even if our ancestors were swordsmen, what can we do?"
- (To Muichiro Tokito about their parents) "You can't trust the word of a man who died trying to do something for someone else. In mother's condition, herbs weren't going to cure her. It was completely foolish. If he hadn't gone out in that storm, only mother would have died. I'm just telling the truth. The "Mu" in "Muichiro" is "Moo"... like a big dumb cow. And this conversation is pointless. Nothing can change the past."