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 had sexual feelings towards Rito and planned to train him as her 'pet'.

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.

Abilities
Nemesis had the ability to transform into anyone (also her attire) and is hinted to have the same ability as Yami and Mea. She also had an incredible strength and easily teleports anywhere.
 * Person Transformation: Before showing her true self, she disguises as Mikan who is watching over Yami, Momo and Rito.
 * Attire Transformation: Interested with Momo's yukata, she transforms her attire into a black-yukata, similar with the ones that Nana's wearing.

Mea Kurosaki
Mea is Nemesis's personal spy whom she used to spy on Yami. Both of them seemed to be very close as Mea is very obidient to her.

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

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

Rito Yuki
She seemed to be very (sexually) interested with Rito. She offers him to be her 'pet'. It is hinted that she likes to torture him. Nemesis also tries to be friends with him, in his refusal.

Momo Bella Deviluke
Nemesis stated that Momo can also be a fun person to torture. Momo also refused to be friends with her.

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.