Why tf do you hate this guy? He is just, a guy? It's so wrong to try and pursue someone you are interested in lol, also he doesn't know she already has someone she likes
Basically the story is in the frustrating alley of non-creepy older male age gap romances: the older guy just can't see himself as a valid love interest for the younger woman. It's the dense MC problem, except here there's an understandable power dynamic and an unspoken tragedy. Our boy's given up on love, but his subordinate loves him. We're rooting for the two to get together.
This guy, by being a more 'age appropriate' male rival who is pushing towards the female lead and telling the male one to back off, is a force that keeps the story in the frustrating alley. He is the better option, as far as our boy is concerned. He's another reason for him to check out, to forget about romance, and to ignore the female lead's advances. She has a better suitor, after all. She shouldn't bother with an old guy like him.
So whenever he shows up and is like "hey, I'm like... handsome and cool and I like you, you know" it's annoying. We want him out of the god damn way already, every moment we spend on him is one we don't need and that isn't one we get with the two leads.