Someone you don't know? You don't recognize Sesus Nagezzer, the most awesome Terrestrial to ever Terrestrial the Terrestrial out of that Terrestrial?
((Also, that is not Roseblack fighting with Mnemon. In the colored version, it is much clearer, since she is wearing red and has black hair. If I had to guess, she would either be Sesus Rafara or Tepet Lisara.))
EDIT: For reference, the colored version:
Spoiler