All I can think of is the word idol. Is it the most correct answer to this question? Or is there any more appropriate word?
In my opinion, idol sounds a bit too "strong".
When someone is a fan of another person, I think it usually means he likes the other one and probably follows his activities etc.
When the other person is his idol, I think it means he idolizes the other person and maybe tries to do everything like him, wear same clothes, etc.
Just my thought..
Example: Uncle John is my girlfriend's _. She likes him a lot.