Here is what Capcom commented about the mysterious blonde character.

As for the girl? Lots of people like to think they know the answer, but nothing's been revealed officially just yet. Might be a returning character, might be completely new, might be something you don't expect at all... but for the time being let your imaginations run wild! I hate to sound like a cop-out, but the game launches in November so you can expect tons of new info within the coming months. In the meantime, take a closer look at the trailer. There's a lot in there that might not be spelled out completely, but really shows how ridiculously awesome this game experience will be!
Source: Capcom comments on the mysterious blonde woman speculation in Resident Evil 6 - National Video Game News |

Either Sherry Birkin or Ashley Graham, but I more leaned towards that the girl is Sherry Birkin. If she is not either one of them, then she must be a new character.