The last "fighting man" that I played in a FPRG was Thurgon.
Like any Burning Wheel PC, Thurgon was built using the lifepath system in the Character Burner. So he has a backstory as a result of that. I also spent build resource on establishing various reputations and affiliations, either directly or as a result of various traits.
The upshot was that Thurgon starts the game aged 29, with B3 Circles (not a bad starting rating), and the following Relationships, Affiliations and Reputations:
Xanthippe (Mother, on family estate)
Aramina (sorceress companion)
+1D aff von Pfizer family
+1D aff Order of the Iron Tower
+1D aff nobility
+1D rep last Knight of the Iron Tower
Because he is Sworn to the Order (a trait), Thurgon also has a fourth Belief (ie in addition to the standard three) that is related to or dictated by his membership of the Order of the Iron Tower.
This is fairly standard stuff for a BW PC. It is what informs the events of play: everything that Thurgon does, over the course of the campaign, is grounded in these elements of the character. The game would not be improved by stripping all this away and having Thurgon be a "generic" fighting man whose backstory is established in the first three levels of play (ignoring the fact that BW is not a level-based game).