Fitz comes aboard the TARDIS after the Doctor murders his mum. No, really. He 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.