When Tohru was a child (in the manga), she got lost trying to run away from a group of boys that were picking on her, unable to find her way home. A boy found her but ran away from her. she pursued him and called out to him to wait and said she wanted to go home. He turned back to her and led her home. He gave her the cap he was wearing and ran off before he could show his face. It turns out that this boy was actually in fact a young Yuki.Yuki always love Tohru and when she said prince Yuki, he blushed and turned into a rat. That is proof he likes her more than anything.
Years later in the present time, Yuki still remembers that incident, but thinks that Tohru has forgotten about it, until he discovers the red cap in her room when they were studying. He asks her why she keeps it to which she responds "Because it's a precious memory of mine! To be able to receive his help...it was really...the happiest moment in my life!" (Chapter 53).
At the beach house in chapter 59 after being confronted by Akito, Yuki wanders off, thinking. Tohru gets concerned when he does not return and goes out to search for him. When she does find him, Yuki comes to realize that her help and kindness have helped him "open the lid" to his feelings. He kisses her forehead and sheds tears as he realizes he loves her. However, he later realizes that he loved her as a mother as he grows to love Machi.
Machi and Yuki met while working in the student council, she being the treasurer and he the president. She piques his interest by characterizing him as a typical, perhaps even lonely, boy while other girls thought of him as a prince. When Yuki and Kakeru Manabe visit Machi's apartment, Yuki learns that because of the pressure put on by her mother to be perfect, Machi finds perfection so disturbing, she has a compulsion to mar anything too orderly, such as trashing neat rooms, walking through newly-fallen snow, and breaking pieces of a new box of chalk. She was the only one who understood Yuki's imperfection.
Machi develops feelings for Yuki, and when Tohru is in the hospital and he needs to reach out to someone, Machi is able to confess to Yuki. Machi later becomes Yuki's girlfriend and eventual wife, and they have a son, Mutsuki Sohma.
Kakeru is Yuki's friend when he met him when Yuki began his presidential work at his high school. Yuki thinks of him as annoying fellow student. Both of them have been labelled as a 'odd couple', Kakeru being the 'clown' of the couple while Yuki, the 'serious' one. Mostly because Yuki thinks Kakeru is a person from a "different world" than him, that he was able to open up to Kakeru. In some way, he changed Yuki and that he was able to smile from the bottom of his heart.
Yuki and Kyo both share mutual hatred towards one another in the beginning of the story. Yuki is envious of how Kyo has the ability to fit into normal people and he is annoyed when Kyo wants to be part of the zodiac meanwhile Yuki wants to escape from the curse. Kyo also envious of Yuki, how he is accepted into the zodiac family, Kyo made a deal with Akito that if Kyo ever beats Yuki, the cat sign will be accepted into the zodiac. Because of this deal, this leads to Kyo's determination to win against Yuki, which causes Yuki to be very annoyed. However, both of their relationship begins to improve after they meet Tohru. Because of her kindness and the two both share mutual feelings for Tohru, they begin to softened their attitudes toward each other and they both cooperate together when Tohru is in trouble. In the end of the original manga, the two called a truce, and each admitted the truth of loving different parts of one another, therefore beginning to understand and respect each other and form a friendship at last. It is hinted that they still remain friendly and frequently contact each other in their adult lives.
Yuki has a twisted relationship of love and hatred towards Akito. Being the Rat of the Zodiac, it placed Yuki at the top of the Zodiac, making him the closest to 'God' or Akito. As such, the two 'love' each other more than the other Zodiacs. However, the twisted mind of Akito that led her to torturing Yuki, made Yuki to hate Akito as a person. But due to the overwhelming feelings from the spirit of Rat that possesses him, Yuki cannot truly hate Akito.
In the Zodiac legend, the Rat rides on the Ox's head to win the Zodiac race. This made the Ox to be labelled as 'stupid' by the rest of the Zodiac animals because it wasn't aware of the rat riding on it. The fooled Ox continued discriminated even after it possessed a human. The current possessed Sohma is none other than Hatsuharu. Tired of being teased, scolded, discriminated because he is the Ox, Hatsuharu went on to hunt down Yuki, who was currently possessed by the Rat, and takes out his (and the Ox's) anger on Yuki (or the Rat), blaming Yuki that he was labelled as a 'stupid' person.
However, Yuki calmly replies that does Haru really think he himself is stupid. When Haru realizes that he should not keep on believing the people around him that he's stupid, he starts to look up to Yuki. Ever since that day of confrontation, Haru has always admired Yuki, even professing a 'love' towards Yuki.
Ayame is Yuki's brother. Yuki seems to hate him but that was because Ayame pushed Yuki aside when Yuki wanted his comfort. Ayame was never really acknowledging his existence and therefore a rift began between the two brothers. Tohru tried to help them but it didn't work and the rift grew bigger. Though as the story goes on, they seem to grow closer.
Fruits Basket Another
Mutsuki is Yuki and Machi's son.