。
I'm fond of Cathy.
Literal
I [topic-は] Cathy [subject-が] take-a-liking-to am.
気に入っている is the ~ている form of 気に入る ('to like, take a liking to') — expressing the resulting state of liking, not the moment of forming the preference. Used for things, places, and people one finds appealing. Note the romantic/affectionate undertone when applied to a person.