So I was on the mangarock app but when I clicked on a manga I had been reading, it got licensed and was removed. I don't even remember what it was called. I do remember some of the plot though:
-main character is a red-headed demon prince guy
-he was sent to earth to fulfill some mission by finding a person who will be a threat to him and the demon realm in the future
-he can collect human emotions with this weird gun/ghost thing
-he meets a girl that is really nice and ends up living with her and her grandmother because he has no where else to live
-he fins out this girl is the person he is looking for and has to steal her soul but is falling for her

Probably Kurenai Ouji

My current top 3:
Fullmetal Alchemist
Pandora Hearts
3-gatsu no Lion