The Sohma Curse is a "bond" between God and the members of the Zodiac. Thirteen Sohmas are possessed by the animals of the Chinese zodiac, including the cat (see Zodiac Legend). Akito Sohma is the current 'god' of the zodiac. Her father, Akira Sohma, was the previous one.
Those who are possessed are born two months premature and transform into their zodiac animal if they are hugged by a person of the opposite sex, or if they are under a great deal of stress (such as exhaustion, sickness or lack of breath). They do not transform into animals if they are hugged by another member of the Zodiac. Their Zodiac animal is attracted to them, and can be used for assistance; for example, rats are attracted to Yuki and he used their help to dig out Tohru's tent from under the landslide in the beginning of the series.
When the Zodiacs see "God" for the first time, they will involuntarily start crying. One of the oldest maids living in the Sohma House once told Yuki's mother, "All of the others started crying too. What else could you call this but a bond?" Akito told Tohru that the Zodiac must all return to her when the cat, Kyo Sohma, is isolated. Another aspect of the curse is that it creates a "dark cloud" over the people that are cursed.
The Cursed Ones
The Rat: Yuki
Yuki Sohma was possessed by the spirit of the Rat. As told in the zodiac legend, the rat is the highest member of the zodiac, and the one closest to god. Akito and Yuki were bound together as children, but only because Yuki was forced to by his mother. For Yuki, Akito is a source of fear. In episode 13 of the anime, it is shown that Akito used to whip Yuki when they were both young.
Yuki Sohma is also one of the few members of the Zodiac who does not live in the Sohma main house. Shigure makes this possible.
The Cat: Kyo
The irregular member of the zodiac, the cat of the Zodiac was the one "left out," as detailed by the zodiac legend. In the Zodiac Legend: Volume 22, the cat was the one who came first to god and was his closest friend. The cat has three forms; human, cat, and true form. The cat's true form is warped and ugly, and smells of rotting flesh. Kyo, the current cat, as well as all past cats, must always wear a bead bracelet made out of human bones and blood to keep from transforming into that hideous state. Kyo has to suffer isolation and rejection, and he blames his horrible fate upon Yuki, because the rat tricked the cat in the legend. The cat is to be isolated after he completes high school, and then the other members of the zodiac are joined in an everlasting banquet until they die.
The Rabbit: Momiji
Momiji Sohma, the rabbit in the zodiac, is a boy and also one of the first members of the zodiac who Tohru meets. He is watched over by Hatori, the Sohma family doctor. He has a little sister, although they are estranged by their father, who separated Momiji from his mother and sister. His mother got her mind erased so she would never remember that she had Momiji or got married to Momiji's father. Momiji's mother wanted to forget because she could not "handle" having her baby turn into a rabbit whenever she held him.
The Ox (Cow): Hatsuharu
Hatsuharu Sohma is the cow; he is also known as 'Black Haru' when he gets into a bad mood. He hated Yuki at first because in the story the rat rode on the ox's back. He said it made the ox look stupid and foolish. However, after meeting Yuki, he has come to admire him. He also has a hatred for Kyo because of their rivalry. Haru also developed feelings for Isuzu Sohma and since he is younger than her by two years, he wanted to wait until his height surpassed hers to tell her that he loved her. The two are currently dating.
The Goat (Ram/Sheep): Hiro
Hiro Sohma is eleven to twelve years old and is the sheep of the Chinese Zodiac, as well as the youngest of the cursed Sohmas. He is depicted as a very intelligent, but verbally aggressive and smart-mouthed sophist who is initially not very likeable. Hiro is the only member of the Zodiac who is shown having a close, loving relationship with both his parents, especially his mother, Satsuki. Hiro also has a younger sister named Hinata.
The Dog: Shigure
Shigure Sohma is a novelist and is also the dog of the zodiac. He is the first member of the zodiac Tohru actually meets, besides Yuki, who goes to her school. He is known to hold back Akito when she gets mad. Shigure also knew that Akito was a woman from the very beginning. He is Akito's lover. Shigure can be very manipulative and makes people feel guilty to get what he wants.
The Boar: Kagura
Kagura Sohma loves Kyo; she was the first to like him for who he is. She is depicted as a very young looking woman, despite being older than both Kyo and Yuki. She has great strength and has a very impulsive behavior, associated with those born in the year of the Boar.
The Seahorse: Hatori
Hatori Sohma is a doctor, also the dragon in the Zodiac. Because there are no more dragons in the world, he transforms into a seahorse, (the Japanese word for seahorse, "tatsu-no-otoshigo," literally means "dragon's illegitimate child."). Shigure took this as a sign that the curse was growing weaker. Hatori is perpetually nonchalant and does not put up with any misbehavior. His talents include hypnotic memory swiping and keeping Ayame Sohma in line. His former lover was Kana Sohma, an extended member of the family who found out about the curse while working for Hatori. When Akito didn't approve of their love she injured Hatori; Kana suffered mentally and had to have her memory erased. Hatori later starts a new relationship with Mayuko Shiraki.
The Tiger: Kisa
Kisa Sohma is a twelve year old girl. She was teased at school for her strange orange hair and orange eyes. Whenever she spoke, her classmates would start to laugh, even if she said nothing funny. This eventually caused her to quit speaking and become extremely shy. She didn't go back to school for a long while, until she met Tohru, who's unconditional love for her made Kisa realize she can be accepted for who she is. She respects Tohru as an older sister figure. She is the Tiger of the Zodiac. Her form is that of a tiger cub, due to the fact that she is still a child. When stressed or angry in her Zodiac form, she has the tendency to bite people.
The Monkey: Ritsu
Ritsu Sohma is the son of the hot springs owner. He is the monkey of the zodiac and acts a lot like his mom. He used to dress in women's clothing to calm himself down, but in the final volume he has started to dress as a man and has cut his hair short.
The Horse: Isuzu "Rin"
Rin is stubborn and was highly determined to find a way to break the Sohma curse in order to free Haru from his suffering. She was angered when she found out that Tohru was looking for a way to break the curse and wanted her to stay out of it. She now accepts Tohru's assistance because of the latter's kindness and grows as protective of her as she is of Hatsuharu, getting angry when someone upsets or hurts her. She appears only in the manga.
The Snake: Ayame
Ayame is Yuki Sohma's flamboyant elder brother. He owns a costume/cosplay shop alongside his assistant, Mine. He is also one of the oldest Zodiac members, knowing that Akito was a girl from the beginning.
The Rooster: Kureno
Kureno's curse was the first to break; even though this information remained a secret from the rest of the zodiac members, the other members always 'sensed' that something was off about him. Akito forced Kureno to stay by her side because she was afraid of being abandoned. He is currently living with Arisa Uotani in the country side. He appears only in the manga.
The God of the Zodiac: Akito
Akito Sohma is a woman (in the manga), but was raised as a man because her mother, Ren Sohma, was jealous of the attention the "god" of the Zodiac was receiving from everyone, especially her husband, Akira Sohma. The only members of the Zodiac who knew Akito's true gender were Kureno, Shigure, Hatori, and Ayame. Akito is very frail and gets sick often. She is short-tempered and acts in an extremely abusive manner to everyone. Akito plays the part of "god" and desperately clings to the assertion that she is the ruler of the Zodiac. After the curse breaks, Akito reveals her true gender to the rest of the Zodiac members. She begins to wear women's clothing and grows her hair down to her shoulders. She is in love with Shigure and the two of them begin living together at the end of the manga.