Can anyone remember the anime?

I've been thinking about it all day, and i really want to find it.
theres a normal kid that goes to high school, and one hari a guy, i think he might be a demon atau a hunter but he has power of maybe shapeshifting atau something, kind of secretly follows him around, and something happens, he might have been attacked oleh a demon, a large one that causes the grey haired man to come save him and tells him he's a demon hunter atau a hunter of some sort, and he can't believe it, but after a litre while he accepts it and hunts demons, anyone remember??