Ada Wong will appear in Resident Evil 6 as a playable character in a separate scenario unlocked after completing the other three story lines.
Confirmation came through a Famitsu interview with the team (and translated by Andriasang). Unlike the other scenarios however, Ada will be flying solo.
Her story still plays into main campaign, as the studio explained that you'll cross over with other players' story lines (if you're playing online) - similar to how pairs of players will join forces at key points in the game.
Capcom are obviously eager to maximise the cooperative and competitive multiplayer elments, as the same interview reveals the inclusion of Agent Hunt mode, letting players control Resident Evil 6's various creatures and enter other people's game. Capcom stressed there was an option to toggle this mode off when playing.