Nemesis

Nemesis, also known as Master Nemesis, is a character introduced in To Love-Ru Darkness. She is a transformation weapon, much like Golden Darkness (Yami) and Mea Kurosaki , but does not share their genetic origins. She is Mea's master, whom she gives orders and uses to observe Yami's changes and recent life. At the beginning of To Love-Ru Darkness, her original plan was to make Yami return to being the heartless assassin she was before arriving on Earth and she convinced Mea that this was the only life they were meant to live as weapons. But after allowing Yami and Mea to pursue and become further swayed by their newfound friendships on Earth, it is revealed that her true objective is to make Yami use a transformation ability called "Darkness", a currently unseen ability unique to Yami. Due to her catalyst-like role in the overall plot, she is often considered the Main Antagonist of To Love-Ru Darkness.

Because she wants Yami to join her and become an assassin again and wants Rito for herself (as her servant) because of his ability to influence Yami and Mea, Nemesis is essentially also the direct antagonist to Momo's planto include Yami, various other girls and herself into a harem for Rito.

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

Appearance
Nemesis is a petite-looking girl with long black hair, golden eyes with vertical silt pupils and tanned-skin. Based on her physical appearance, Rito had mistaken her as a little girl. However, she can change her appearance into anything she desires, such as a crow or a cat, or change her herself into a well-endowed adult woman. She has taken to giving herself cat ears often.

At first she wore a scanty black dress, but she used her abilities to change it into a black yukata (claiming how effective it is in a fight) with a  star shaped gap symbol (similar to the one Yami and Mea have on the chest of their black Gothic clothing) and  she has yellow sash wrapped around her waist.

Personality
Nemesis is sadistic, mischievous, domineering and mysterious. S he believes that a transformation weapon's only purpose for living is killing. She is often seen hiding in the shadows observing Rito, his friends and their daily lives through Mea until To Love-Ru Chapter 23-24, where she begins to approach them herself. She also has an eery and erotic side to her and finds Rito's shyness arousing and would like to have him as her boy toy. She said that she doesn't have full control over her erotic personality and would act on impulse. Upon her first appearance, her erotic tendencies quickly earned an uneasy impression from Rito, and a strong dislike from Momo.

She can be quite patient, like instead of persuading Yami to return to her  old heartless life, she decides to wait until she uses the "Darkness" ability, believing that Yami will return to her real path naturally and that the "Darkness" will inevitably occur regardless of Yami's relationships on Earth.

History
After the somewhat unknown tragic failure of "Project Nemesis" Nemesis said to be the only survivor. Afterwards, she came to the fallen organization where she found  Mea and decided to raised her. She taught her the purpose of a Transformation Weapon and about her sister, Golden Darkness, whom they came to Earth to bring back into space as who she was before.

Powers and Abilities
Nemesis has the same (maybe stronger) transformation ability as Yami and Mea. She also has incredible strength and easily teleports anywhere. She has also shown to have great strength, although the full extent of this is unknown.
 * Person and Animal Transformation: Before showing her true self, she disguised as Mikan when she approached Yami, Momo and Rito. Nemesis can also alter her appearance and reshape her body. She can also transform into animals, even those much smaller than herself, such as a crow or a cat. This ability is something that not even Yami or Mea are seen 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). It is also unknown if she has a limit to how long she can hold said form.
 * Vocal Alteration/Imitation: 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 is able to transform her clothing, as seen when she switches between a black nightgown and a yukata. She is also seen wearing a maid outfit in Chapter 31.
 * Mental Manipulation/Body Jack: It is heavily implied, if not confirmed, that she is able to briefly hijack the bodies of lesser-minded individuals even if they were conscious beforehand. While under such effects, her victims have the appearance of zombies and borrow a fraction of her strength in order to hold their own in battle. It is also shown that she is able to speak through them, as she speaks through Saruyama in order to communicate with Yami.
 * Telepathy: She also seems to be capable of short-range telepathy, which she mainly utilizes to give orders to Mea discretely. However, until recently (To Love-Ru Darkness chapter 31), it wasn't extremely clear that she was capable of this, and it is entirely possible that she only uses this power with Mea.

Trivia

 * It was hinted that she was the one who had the demon sword Bladix end up at the Tenjouin mansion, to show Yami the true nature of a weapon is to fight and injuring others (To Love-Ru Darkness Chapter 34).