Jiji is Kiki's companion as she is training to become a full-fledged witch. One time, he has to pretend to be a stuffed cat for a boy's birthday gift. The real stuffed cat which looked exactly like him, was lost in the forest and Kiki had to go find it. This was during Kiki's first delivery.
Jiji also gets a girlfriend, a white cat named Lily. In the credits, you see that Jiji and Lily had kittens.
In the American version, Jiji is a chatty and sarcastic cat that likes to act proud. In the Japanese version, Jiji is a cautious and cute cat that acts humble and is always ready to help Kiki.