When he was a child, Kazuma was mean to his grandfather, who was then the member of the Soma family possessed by the cat of the Chinese Zodiac, during their first and only meeting simply because his grandfather was cursed by the cat. Hoping to make amends for how he had treated his grandfather, he decided to take care of the young Kyo Soma

Spoiler

after Kyo's mother had committed suicide.

He is Kyo's martial arts teacher, but is also an adoptive father to him. Kazuma thinks well of Tohru and hopes that her love for Kyo is sincere and that her relationship with Kyo will not mirror that of his grandfather and grandmother. Kyo calls him "Shishou" which means "master". Kazuma has taught many of the other Somas martial arts, including Kagura, Yuki and Haru. (Source: Wikipedia)