be interested in someone—be keen to know or learn about a person:
- I do not suppose that George was interested in Stevenson as a writing man.
have an interest in someone—concern oneself about or sympathize with smb.:
- She happens to be my only sister’s child and I have an interest in her.
Note: Neither expression correlates in meaning with the phrase interest oneself for someone—seek attention for another person:
- I will interest myself for you and try to obtain a promise of the post.