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The Handsome Rat of the Sohmas
This handsome Sohma is a member of the school's student council and very popular amongst the other students. A friendly and polite young man on the surface, Yuki lacks self-esteem and has an inferiority complex. While Yuki does have the desire to make friends with others, he lacks the self-confidence to try becoming more than an acquaintance to others. He also avoids getting in intimate situations with female admirers lest his transformation into a rat occurs. The presence of Tohru in his life allowed Yuki to slowly come out of his shell with being close to others.
Yuki faced a harsh past thanks to his upbringing as a cursed Sohma. As a child, Akito kept him isolated from the other Sohma and would physically and mentally torture the boy having him believe that no outsider could accept him because of his curse. In addition when he was playing with some children at one time, one of them accidentally caused him to transform which led Akito to make the order to Hatori to erase their memories of Yuki.
In regards to his rivalry with Kyo, Yuki is disgusted of him because of the boy's desire to want to be accepted by the Sohmas. Yuki is envious of Kyo because of how he can express how he feels and believes Kyo could be capable of living a life free from the Sohmas if he improved his social skills.
Like Kyo, Kagura, and Haru, Yuki was trained in the martial arts by Kazuma Sohma. He is adept enough where he can effortlessly best Haru and Kyo. He also suffers from frequent asthma attacks as a result of having weak bronchial tubes.
Background As a child, Yuki was traumatized by Akito Sohma, the leader of the Sohma family. When Yuki was younger, Akito used to have total control over him. She would tell him terrible things that he could never forget, like that he was the most hated member of the family, and that if it weren't for her, Yuki would have no reason to exist. These statements were seemingly confirmed by the behavior of the rest of the family: Hatsuharu proclaimed he hated Yuki (although he later comes to adore Yuki), despite the fact he had never met him before. His brother, Ayame Sohma, brushed off his cry for help, and Kyo always hated and blamed him. Everyone else in the zodiac did not speak with him, though that was probably not an intentional snub on their part. On top of being mentally abusive, Akito was often physically abusive as well, and sometimes would lock Yuki away inside a dark room to torment him. As a result of the combined abuse, Yuki has terribly low self-esteem, and suffers from self-hatred and an inferiority complex. Though his outward demeanor shows nothing of his childhood torture, Yuki reveals to Tohru that he does not care or appreciate who he is as his personality was shaped to please others and conform to society, which he incidentally excels at.
Yuki's worst enemy would be Kyo Sohma, the cat of the zodiac. Hatsuharu Sohma felt animosity towards Yuki during part of their childhood because of the story often told to Haru about the rat taking advantage of the stupid cow; this made Haru develop an inferiority complex towards his older cousin. However, after actually meeting Yuki face-to-face, Haru was able to see Yuki's extreme kindness and developed a deep admiration for him.
Yuki also feels a close connection with his cousin Kisa Sohma, as Kisa was teased because the children in her class thought she was different from others. Until Tohru Honda touched her heart, she didn't go to school and she couldn't speak, much like Yuki back when he was younger. Likewise Yuki, despite his popularity, rarely interacted with anyone on a personal level and was extremely private and introverted. Until he met Tohru, Yuki was afraid of becoming close to anyone because of an incident during his childhood when his secret was discovered by his friends during a game of tag, when Akito ordered Hatori Sohma to erase the children's memories, thus leaving Yuki alone and without friends. After meeting Tohru, Yuki started warming up to other people and becomes much more social, as well as accepting who he really is.
Yuki also appears to have respiratory problems, due to his weak bronchial tubes, and gets regular checkups from Hatori to monitor his condition. In his childhood these asthma attacks were apparently frequent, though they appear to be less so in the present. Still, he has to take extra precautions because he occasionally has attacks, which could cause him to transform in public. Throughout the manga, the attacks are less and less frequent.
After Kyo was freed from the curse, a series of events was triggered and the rest of the cursed Sohma family members became free. Yuki is the last Sohma member to be freed, noticing this as he was talking to Machi. The spirit of the zodiac took the rat spirit and thanked Yuki for keeping his promise for a long time. It is possible perhaps Yuki was the last one to be freed because he was the rat, the first of the Jyuunishi.