What is his deal with me?

One of my best friends, her brother, has a friend who tends to “pick on me” and his behavior is a bit odd. I was pet sitting/house sitting for her for 3 months recently and he would be over a lot. In the beginning he was an ass towards me, but got nicer towards me. I would catch him looking at me while talking to her brother and their friends. He also will mock what i say, one time he jumped out from behind the wall in the living room when i got back from work. One time i told him he looked “nice” and his face got red it seemed like it boosted his ego. I have not seen him since December but im wondering how i can try to get at him..
What is his deal with me?
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