.....wow,I'm pretty sure I've read hundreds of manga just like this one.
OP MC : check
He has a Mysterious power : check
he only has one parent or none : check
weird hairstyle : check
a harem: check
he already met the heroin when they were young : check
and they made some kind of promise: check
and only the heroin remembers it : check
they meet a strong monster but turns out he's the weakest of the bunch in the next chapters: check
what else?