I don't think there's any reason to believe that any characters will be faction-exclusive. What's a lot more likely is that all five factions will factor heavily into the story, and be roughly equally represented in the game's
As I suspected, this video confirms some of the rewards of doing well in Faction MP to include unlocking alternate costumes for characters. In the other video, Brian Goodman says that NRS is taking a lot of steps to prevent gaming the Faction system. So again, they obviously don't want everyone coming out of the story mode going "wow all the main characters were White Lotus, so I'm gonna play that faction!"