Momoko Manual

Momoko is a girl who was born in a witch family in which most of its female members possess magics. Yet they can only reveal their magical capability to "the one" i.e. their soul mate / spouse. The more magic she use in front of strangers, the more parts of her body would become cat-ish (such as cat ears and whiskers), and if any stranger learns her true identity she would turn into a cat completely -- like her mother. Having joined the magic club in her school, Momoko has used her ability too often that her cat ears pop out every once in a while so she must find "the one" as soon as possible, but her candidates have already become suspicious.

(Source: Anime News Network)