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. Nemesis appears to somewhat care for Mea, and she wants Mea, Yami, and herself to be together in the end, but because of her domineering personality, she is not above keeping secrets and using and manipulating others to her own ends, not even Mea.
In Darkness Chapter 40, Nemesis takes over Mea's body and claims to everyone else that Mea is an alternate personality that Nemesis created. But to Mea, she reveals that she was in instable condition and needed to temporarily fuse with Mea's body to keep existing. Mea is still in control of her personality, but Nemesis is in control of her body. Nemesis wishes for Yami, in her Darkness form, to finish her mission to kill Rito, and then she will remove herself from Mea, and the three living weapons will be together, but until then, she keeps her control of Mea's body against Mea's will.
Mea manages to break free from Nemesis' control and expelled Nemesis from her body. Mea bears no hatred towards Nemesis and is still grateful to her for being there when Mea was alone, but has become more independent from her former master, which she expresses by calling her "Neme-chan", which surprises her.
Yami is a transformation weapon like Nemesis, and the center of Nemesis' plans. Nemesis and Yami seem to know each other, though they meet each other for the first time in Chapter 23-24. Nemesis wants Yami to return to her old life as an assassin. She plans to make Yami use a transformation ability called "Darkness" (an ability unique to Yami), and with it, she believes that Yami will naturally revert back to her old self. Once Yami becomes Darkness and finishes off Rito, the three living weapons will be together. But after Yami's Darkness transformation is defeated by Rito, it is now unknown if Nemesis has anymore plans with Yami.
Nemesis and Tearju seem to know about each other, but never met each other in the past. Nemesis states that Tearju is the "barrier" to her and Yami. The two officially meet in Darkness Chapter 40.
Originally Nemesis planned on having Yami finish her previous job to kill Rito, until she notices how he 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.
While Nemesis enjoys toying with Rito in her spare time, she seems to still maintain her plan to have Yami transform into Darkness and finally kill Rito. But after Rito and his friends manage to defeat Yami's Darkness transformation and return her to normal, and now that Mea is no longer subservient to her, she now calls Rito her only servant and continues to go to him for mischief and amusement.
After she battles King Gid Lucione Deviluke in her attempt to begin a new galactic war, Nemesis was left in fatal condition, fading away into nothing. However, Rito, out of compassion and the belief that she has the capacity to change like Yami and Mea, offered to let her live inside his body until she mostly recovered. Believing that he has no obligation to help her, Nemesis finds Rito's willingness to save her and trust her with his own body very perplexing, but nevertheless accepts his offer, which she used to survive for future entertainment. Despite her mischievous behavior in Rito's body, she showed some slight respect towards his charity and did not refer to him as her "servant" and treated him more as a friend/roommate. She also refers to him by his first name ever since she started living in his body, and still does after leaving. She now refers to him and Mea as her "ex-servants".
After the fight against Kuro, Nemesis admits to Yami that she saved Rito because Rito is an interesting fellow. Later on in Chapter 75, Nemesis reveals that she loves Rito. It's unknown if she actually love him or if it's just part of her plan.
On first meeting, Nemesis plays with Momo's tail weakness in front of Rito and decides that she is worth torturing as well. They both share erotic and mischievous tendencies especially towards Rito, something they can relate to, but her's are so strong and sadistic, 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, other than Nana as seen on several occasions. 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.
Momo generally dislikes Nemesis, and Nemesis scarcely misses the opportunity to tease Momo. But their relationship is not all antagonistic. After a fierce battle against King Gid Lucione Deviluke left her slowly fading away, Nemesis appears to show appreciation for the fun she had with Momo, and Momo thought to herself that despite circumstances, she did not want things between them to end with Nemesis disappearing.