Fitz was one of the longest serving companions of the Eighth Doctor in the Eighth Doctor Adventures and was portrayed by Matt Di Anglo in a Big Finish story. I seem to recall reading somewhere that he was a notherner in a leather jacket? Sure, like they'd let someone like that on a TARDIS.
Fitz started off as a kind of sad-sack loser before maturing into a kick-ass companion. He had all kinds of terrible things happen to him, including multiple deaths, and spending a millennium separated from the rest of the TARDIS crew, as a sort of fake version of himself created by someone else's memories of him, before somehow magically getting turned back into the real Fitz.
He had some great speeches, like the one about how he remembers his time in plastic even though he knows that's not possible, or the one he gave in Italy about how the Doctor is dangerous because, without even trying to, he makes his companions want to be like him. Stephen Moffat definitely never read any of the books with Fitz in them. You can tell because Fitz didn't fall into a crack in time and get forgotten by his girlfriend, his girlfriend walked into a crack in time and got forgotten by him. Totally different head, totally.