http://www.mangago.me/read-manga/homunculus/
mystery yes. u will definetely think. likeable nahh, memorable yea. absurdity: tons but not humor, more like irony. romance nah,but there is a ship i guess:'). seinen.
http://www.mangago.me/read-manga/monster/ really good mystery. u will think a lot(morals). the main chara is likeable the vilain is memorable. there is suspense here but the story is slow paced. romance nah, there is a ship(??i never thought about it). seinen.
villain*
Monster or Death Parade. Death Parade is an anime-only though, but it fit your description so well.
Can you recommend me some manga (it doesn't have to be from this site)
I like:
-A good mystery or ongoing theme/goal
-Something that will make you think
-Likable and memorable characters (unless the plot is just *that* good the characters don't matter)
-A bit of absurdity (whether it be in the plot, characters or humor)
-Maybe some romance? It depends (or at least a good ship)
-Suspense
-More Shounen then Shoujou