Ren Kaiken
Ren is an assassin from the Conquest arc. She has green hair and purple eyes, and wears all black, including a cloak with a hood and cloth "mask" to conceal her appearance.
History
Ren started her life in Greenleaf before running away to Yaksha and training to be an assassin. Since then, she had many prominent assassinations, including being hired by Shiryoku to kill Han'you of Greenleaf.
Lucanus hired her to assassinate Sabaku, but she wound up killing Alphonse instead. However, she was still hired by The Ghost Lord to kill Ammy and steal the Lunar Pendant.
When she attempts to do so, the Lunar Pendant's defense mechanism activates and destroys her body but seals her soul inside of it.
She ultimately befriends the entity within the Lunar Pendant and allows it to absorb her soul to save it and Ammy.