There's no "Harem" tag on this title. And MC is not OP. While he does have a good skill, it's been an absolute grind-fest for both him and the girl.
Also, spoiler alert, but... the girl's "friends" (read as: Most Popular Boy and Girl) were turned into zombies and subsequently killed. So we have that deviation from the typical trope of them being around forever and ever to be an absolute nuisance and pains in the ass.