Female rapper+Aesop Rock. Who was she?

Saturday night I was at the Atmosphere concert in Berkley. For his final song Aesop Rock did a duet dedicated to Eyedea, with this chick who was so good. Who is she ? It wasn't clear, but it sounded like she was British. I want to check her out, and I I'd love to know the name of the song, too. Thanks!