I'm looking for a manga where a demon (when I say demon I mean an actual demon monster not a character that looks human) falls in love with a human girl.

Mahoutsukai no Yome is exactly what you are looking for, well, it's unknown what exactly the monster character is, so I can't confirm if he's a demon, but his head looks like somekind of deer skull and his eyes are red with black sclera which make him look pretty demonic, I've only watched the anime but if the manga is as good as the anime it's a awesome read

There is also Yuusha wa Shimei wo Wasureteru, I haven't read this one as well but I want to (I was recommended it just a few days ago in another thread), by it's description it seems that the protagonist girl and a villain demon fell in love with each other