Usually the protagonist. However, there's a thriller novel by Jack Higgins called Solo I first read as a teen and it stuck with me. The reason:  Higgins gives you the back story of both protagonist and antagonist equally, they are both sympathetic and flawed and one isn't really the hero and the other isn't a villian, and yet they're headed for a collision because of an error in judgement that one of them doesn't realise the severity of. Absolutely brilliant, stayed with me, and held up on re-read.

Another like this is Grandmaster by Warren Murphy and Molly Cochran.