Why does this guy act so strange when I come around and he’s with his girlfriend?

so there’s this guy at my school who i recently found out finds me attractive and has a “small” crush on me all while he has a girlfriend :/. whenever he sees me and he’s with her, he tries to hide from me or avoid eye contact by pulling out his phone and putting away when im out of sight. i’m so confused on why does he do this? Is he ashamed of smth or her? i don’t get it since his girlfriend is so pretty. what is the deal with him?

Why does this guy act so strange when I come around and he’s with his girlfriend?
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