In the new story trailer, we see Ada take on a very dark side and appear to play the role of the game's villain. At first I wasn't sure if it was her but if you notice, she ALWAYS has on a red scarf regardless of the outfit. Ada's signature colour is red so I think this is a dead giveaway that it's her (in addition to the hair style and Asianish features).
Although Ada's motives have been unclear in all of her appearances so far, she never did anything outright "evil." At rare moments you also see her show care and compassion for the protagonists which is something Capcom has never allowed the "villain" to do in RE.