Once you’ve completed the campaigns of Chris, Leon and Jake in Resident Evil 6, you’re given access to a fourth playable campaign—that of Ada Wong’s, which helps tie some of the game’s plot threads together.
Unlike the other three, Ada’s campaign can only be played solo, and has no option for co-op play with a partner. This is about to change soon—Capcom have announced that they will be releasing a free update that lets you play Ada’s campaign with a friend. The update will also make her campaign available from the very beginning of the game, so you won’t need to unlock it in order to play it.
Additionally, another incoming free update will add a No Hope difficulty mode. We’ll have more news on this as Capcom make it available.
Food for thought:
Capcom have also clarified that any “on-disc DLC” in Resident Evil 6 is meant to be free future DLC. No paid downloadable content is on the game disc.