In the beginning, there were only two characters in Musume: chaos witch Suzu Aoyama and axe fighter Mayu Suzuki. Suzu has been on this site from the very beginning. Mayu, however, has just shown up.
I've had art of Mayu (drawn by a different artist) on the site before, but ultimately I scrapped that art and asked concept artist Xesxus to build her from scratch. This is the result, and I think it's amazing. Lots of people seem to agree.
So who is she? Well, she is basically Suzu's best friend and her exact opposite. She is as strong as Suzu is fast, but is impulsive and carefree where Suzu is studious and reserved. Opposites attract, and in Musume, the Suzu/Mayu combination will lead to a lot of drama and dead monsters.