Nemesis

Also known as Master Nemesis. Nemesis is a sadistic, demanding girl who is responsible in giving orders to Mea. She uses Mea as her spy to look after Yami's changes and recent life. In addition, Nemesis is the one behind the plans to make Yami return to her emotionless-assassin's life again. Claiming that she knows that one day, Yami will return to her real path naturally, she decided to live in the human world and tries to be friends with everyone; in refusal of Momo and Rito. Like the other girls, she also seemed to have sexual feelings towards Rito and planned to train him as her 'pet'.

Her real name is unknown, and the name 'Nemesis' is based from Project Nemesis, a weapon project that is organized by the assasin organization who created both Yami and Mea. However, this project, as stated by Tearju, is a failure and Nemesis is a survivor from the project.

Appearance
A petite looking girl with long black hair, golden eyes and also tanned-skin. Based from the physical appearance, Rito had mistaken her as a little girl. However, she can change her appearance into anything she desired, such as being a crow to spy on Mea everyday or changes her human appearance into a more mature way. She has taken to giving herself cat ears often.

Abilities
Nemesis had the ability to transform into anyone (also her attire) and is hinted to have the same (and maybe stronger) ability as Yami and Mea. She also had an incredible strength and easily teleports anywhere.
 * Person and Animal Transformation: Before showing her true self, she disguises as Mikan who is watching over Yami, Momo and Rito. Nemesis can also alter her appearance and reshape her body. She can also transform into a crow and a cat. This ability is something that not even Yami or Mea are capable of doing (although Yami has shown to change her body measurements and size, grow wings and turn her legs into a fish tail similar to a mermaid, but never transformed her entire body into an animal).
 * Vocal Transformation: In addition to her shape shifting abilities, she can also change her voice to sound just like anyone.
 * Hair Transformation: Like Yami and Mea, Nemesis can also control the movement of her hair and transform it into any animate object like a key. It is yet unknown what other kind of transformation she can create with her hair.
 * Attire Transformation: She has been able to change her clothing, as seen when she switches between a black nightgown and a yukata.

Mea Kurosaki
Mea is Nemesis's personal spy whom she used to spy on Yami. Both she and Mea share a close relationship. Mea is very obedient to her as she is the first person to confront her after the end of the Galactic War. However, it is not known how much Nemesis truly cares for Mea. Although she ultimately permits Mea to pursue her friendships on Earth, Nemesis previously persuaded Mea that her nature as a weapon is to ultimately be alone. And Nemesis seems to choose not to inform everything she knows to Mea, such as the "Darkness" inside Golden Darkness.

Yami (Golden Darkness)
Yami and Nemesis seem to know each other. She tries to befriend and get closer with Yami again.

Tearju Lunatique
She seemed to know who Tearju is, by stating that she is the 'barrier' to her and Yami.

Rito Yuuki
Nemesis is aware that Rito is the human who draws both Yami and Mea to humanity. As such, to observe more closely how their lives progress, she decides to become friendly with him, in her own erotic way.

She seems to be very (sexually) interested with Rito, and finds his nervous expressions towards her arousing. In her first appearance, she offered to train him to be her 'pet'. She playfully considers him as her servant, and essentially enjoys treating him as her boy toy.

Momo Velia Deviluke
On first meeting, Nemesis plays with Momo's tail weakness in front of Rito and decides that she is worth torturing as well. Her erotic and mischievous tendencies are so strong, even Momo has considered her a threat. Despite Momo's plan to build a harem of girls around Rito, Nemesis is apparently the only female whom Momo does not want near Rito, and apparently the only one who can stir up feelings of jealousy in Momo. Nemesis very much now enjoys getting on Momo's nerves, and Momo considers Nemesis her natural enemy. She even told Momo to give up on the harem plan because she wants Rito all to herself as her servant.

Story Plot
She is known to give orders to Mea to do anything she desires.

Nemesis was originally mentioned by Mea in the manga until chapter 23 where she appeared in front of Yami, Rito and Momo.