The fifth and final new fighter to be added to the upcoming Ultra Street Fighter IV will be Decapre. She's new, while also being not new. It's weird.
She's not a new character, having first appeared briefly in Street Fighter Alpha 3 as an NPC (which you can see in the intro to the video below). But she is new as a playable fighter, and has a pretty great name, though I will now and forever know her as Evil Cammy.
Here's Capcom's excited description: