Originally an ASCII art character from 2channel, but now has been immortalized in fanarts too. Looks like :3 + o_o.
He is sort of like an actor who starred in a wide variety of "Yaruo-threads(やる夫スレ)". It is a type of online story / fanfic / novel threads on 2ch. His role varies according to the story he starred in. If you want to get an understanding of Yaruo fanart, you need to know what kind of story he's starring in.
