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.

Believing that Yami will return to her real path naturally, and that the "Darkness" will inevitably occur regardless of Yami's relationships on Earth, Nemesis decides to reveal herself and approach the other characters in person to observe their lives more closely. Upon her introduction, her eery, erotic and domineering personality immediately earned an uneasy impression from Rito Yuuki, and a strong dislike from Momo Velia Deviluke. Because she wants Yami to join her and become an assassin again, and wants Rito for herself because of his ability to influence Yami and Mea, Nemesis is essentially the direct antagonist to Momo's planto include Yami, various other girls, and herself into a harem around Rito.

Her real name is unknown, and the name 'Nemesis' is based from Project Nemesis, a weapon project that was organized by the assasin 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 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.

Abilities
Nemesis has the same (maybe stronger) transformation ability as Yami and Mea. She also has incredible strength and easily teleports anywhere.
 * 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 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 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: 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.

Mea Kurosaki
Mea is another transformation weapon and Nemesis' personal spy whom she used to watch over 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 heartless and alone. And Nemesis seems to choose not to inform everything she knows to Mea, such as the true nature of the "Darkness" ability inside Yami.

Yami (Golden Darkness)
Yami is a transformation weapon like Nemesis, and the center of Nemesis' plans. Nemesis and Yami seem to know each other, though have only met each other once (To Love-Ru Darkness chapters 23-24). Nemesis originally planned for Yami to return to her old life as an assasin, but revealed that her true objective is to make Yami use a transformation ability called "Darkness", an ability unique to Yami.

Tearju Lunatique
Nemesis and Tearju seem to know about each other, but unlikely have met each other in the past. Nemesis states that Tearju 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.