Apparently, having six playable characters and three different campaigns isn't enough for Capcom. Data-mining internet sleuths have pored over code from the recently-released Resident Evil 6 demo, and have uncovered references to a fourth playable campaign.
It appears Ada may have her own campaign. The red dress-wearing villain has been a franchise favorite, and she's made numerous appearances in screens and trailers for the game. According to the decrypted data, Ada will be joined by another co-op partner, presumably Carla.
seg000:0018D280 0000000E C PLAYER_HELPER
seg000:0018D290 0000000B C PLAYER_ADA
seg000:0018D29C 0000000E C PLAYER_SHERRY
seg000:0018D2AC 0000000C C PLAYER_JAKE
seg000:0018D2B8 0000000D C PLAYER_PIERS
seg000:0018D2C8 0000000D C PLAYER_CHRIS
seg000:0018D2D8 0000000E C PLAYER_HELENA
seg000:0018D2E8 0000000C C PLAYER_LEON
seg000:0009E4F4 0000000A C uPl00Leon
seg000:0009E500 0000000C C uPl01Helena
seg000:0009E50C 0000000B C uPl02Chris
seg000:0009E518 0000000B C uPl03Piers
seg000:0009E524 0000000A C uPl04Jake
seg000:0009E530 0000000C C uPl05Sherry
seg000:0009E53C 00000009 C uPl06Ada
seg000:0009E548 0000000C C uPl07Helper
Resident Evil 6's Japanese website currently teases a fourth main character for the game. It seems likely that this newly-uncovered Ada evidence fits the bill.
"While it is correct that at E3 we confirmed that Ada Wong would feature in Resident Evil 6, we have no comment on the reports that are currently circulating," a Capcom spokesperson told Eurogamer.
Resident Evil 6 wouldn't be the first time Ada would be playable in the franchise. She also had a playable campaign in the PS2, Wii, PS3, and Xbox 360 versions of Resident Evil 4. In that game, she had a grappling hook--a feature that's likely to return in RE6.