No robin never does find out who slade realy is. In the comic books slade and robin never have this "arch nemisis" thing going on so i think the whole point of slade looking like robin in the show is to say that robin is afraid to end up like slade. Its just a part of his personality. Hope this helped a little.

I think Slade is related to Robin some how because in season 1 episode 13 robin kicks off half of Slade's mask.Reveling dark hair and Robin is like Slade in some what of a matter. That is what I think about Slade and Robin.