Kuroko Koumori

Kuroko Koumori is the main protagonist of the manga Murcielago. Formerly a serial killer, kuroko becomes an assassin who's given the task of eliminating targets that contribute to the high levels of crime in the city.

Murcielago
Since her childhood, Kuroko had murdered approximately 715 people before being apprehended by the police. While she was awaiting her execution, the police department proposes an ultimatum to her; as she was a mass murderer, she could be used to expose of mass murderers who were unstoppable as far as the department was concerned. As such, Kuroko avoids getting executed for her crimes, and she becomes a hitwoman who tracks down and kills mass murderers under the police department's orders.

Hinako, a high school girl, partners up with Kuroko on assignments. In a throwaway comment, Hinako reveals that she had "terrible" parents that were killed in the past by Kuroko. Kuroko agreed to become an assassin in part, because it would satisfy her blood lust, taking sadistic pleasure in massacring her targets. For example, during the school bombing arc, she manipulates a student into killing the culprit, and sexually assaulting a bully. When she's not busy exterminating targets, Koumori indulges in having sexual encounters with women, as well as searching for women on dating sites. The closest thing that she has for a lover is Chiyo Yanaoka, daughter of the boss of the city's Yakuza. While she insists that she isn't a lesbian, she states that Koumori is the only exception to that rule.