As long as he has a connection to Diaz, Oliver will want to be a friend to him.
This new villain may be completely unconnected to anybody Oliver has fought before.
Unfortunately, Oliver doesn’t have more than “The Demon” to go on.
Naturally, that title calls the al Ghul family to mind, but Ra’s al Ghul is dead, and neither Nyssa nor Talia would be housed in a men’s prison, if indeed a normal prison could hold them.