Othello is a manga by Satomi Ikezawa in 7 tankoubans.
Yaya is a shy, quiet teen with a passion for singing, J-rock, gothic lolita, cosplay and music in general. But she has a secret: When she looks in a mirror, or hits her head, Yaya's second personality, aptly named Nana, takes over her body and puts the wrong-doers in their places, shouting her catchphrase "Justice is done!"
The series chronicles Yaya and Nana's somewhat episodic adventures against bullies, perverts and generally mean people. The manga also has a lot to do with music, as several of the characters have connections to the music industry or play(ed) in a band.