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, Jiji and Lily have kittens.
In the original japanese script, Jiji is actually female.
Also a difference in the original japanese, it is left ambiguous as to whether Jiji ever regained the ability to speak with Kiki.