Unlike her sister, Yami, Mea has never known anything in her life other than being a living weapon. Her first memories are of waking up in a destroyed life pod and wandering throughout the rubble of the destroyed research station she was born in and meeting Nemesis, a living weapon from another project. Nemesis raised Mea to be a killer and that people like her have no place with humanity. Mea and Nemesis came to Earth to bring Yami back into space as the cold-hearted assassin again. At the beginning of To Love-Ru Darkness, Mea enrolled in Sainan High to watch over Yami and introduces herself to her as the next generation Transformation weapon, who can be considered Yami's little sister. While at Sainan High, she becomes the first friend of Nana Aster Deviluke, who enrolled with along with Momo and Yami.
While living on Earth and attending school with Nana, Mea maintains the conviction she learned from her master Nemesis that she is really a heartless living weapon, and believes that her usual cheerfulness and mischief are only a facade. However, maintaining her friendly behavior and being around friends at school slowly make her open herself up more to others. At school, she becomes acquainted with Nana's companions, her sister Momo, and Rito, Yami's supposed target. While investigating Rito, she discovers that Momo plans on adding Yami into a harem she is building for Rito, which goes against her plan to bring Yami back into space as an assassin again. As she continues to observe Yami, she becomes uneasy and confused with Yami's growing relationships with others. Yami soon reveals to Rito and Momo that Mea is her little sister, and she encourages Mea to open up to others.
Dr. Tearju Lunatique arrives to Earth and begins teaching at their school, still caring for Yami and hoping to reunite with her. Although Tearju is both Yami's and Mea's creator, she has never actually met Mea in person until her arrival at their school. Because Mea is essentially Yami's little sister, Tearju was quickly ready to consider Mea her little sister as well. However, Mea rejects her due to her loyalty to Nemesis and their plan to bring Yami back to their lives as killers.
Mea becomes very confused by her sister's choice to remain on earth living a peaceful life surrounded by friends, and though she does slowly come around to the idea, she maintains her desire for her sister to join her and become an assassin again. She stubbornly convinces herself that deep down, she is the heartless weapon Nemesis claim they truly are. But when Rito questions her master's teachings, Mea takes control of Rito with her Psycho-dive ability, and forces him to lick her body as punishment. However, she unintentionally shared with him her deeper feelings. When Rito discovers and reveals that deep down she is actually lost, confused and lonely, she angrily denies those feelings and insists that she is a heartless weapon.
Nana eventually witnesses Mea using her transformation powers in a fight and discovers that she is a Transformation Weapon like Golden Darkness, much to her shock. Mea then tells her that their friendship was fake and that they should stop pretending to be friends. She then leaves, leaving Nana heartbroken and seemingly cutting her ties with Nana. But when Nana approaches her again to restore their friendship, Mea was moved by her feelings and realizes that she cares for Nana too, and they have since reconciled and become friends again. Nemesis is aware and aproves of their relationship and permits Mea to think more independently. Mea has since become more open around her friends.
Mea initially keeps her identity as a transformation weapon a secret because she wants to hold onto the life and friendships she earned as Mea Kurosaki. However, in Darkness Chapter 37, everyone finds out her true identity when she uses her powers to save the school from a micro black hole. Yami introduces her to everyone as her little sister, and everyone welcomes her, not caring she's a transformation weapon.
In Darkness Chapter 39, just when Yami and Mea felt accepted by everyone and accepted their new peaceful lives, Yami's "Darkness" transformation activates, and Mea transforms and becomes Nemesis. Nemesis explains that Mea is a seperate personality that Nemesis created in order to better adapt being around other people. Because Mea has always appeared to see Nemesis face to face before, she was not aware that she and Nemesis were two minds in the same body, and Mea's personality is currently being supressed by the darkness inside Nemesis.
As the newly transformed Yami continues to go after Rito and cause all kinds of perverted chaos, Mea speaks to Nemesis inside her subconscious, insisting that Nemesis lied about Mea being her alternate personality, and demands her body back. Nemesis explains that when they first met, she was in a physically unstable condition and needed a physical vessel to stabilize herself, and so she fused herself into Mea. She plans to leave Mea's body after Darkness kills Rito, and then the three of them will be together. Mea attempts to persuade her from allowing Rito to die, but Nemesis remains unpersuaded.
While Mea cannot manage to regain control of her body by herself, she manages to gain control of a single hair, which she used to call Oshizu via psycho-dive. With Oshizu restraining Nemesis, Mea expresses her respect and gratitude to Nemesis. After she is free from Nemesis, she decides to stop Yami from killing Rito, but she cheerfully calls Nemesis "Neme-chan" before she leaves, much to Nemesis' surprise.
Mea reunites with Nana and Momo, and deduces that Yami's obsession with Rito and newly perverted tendencies are caused by a bug inside her as a result of Rito's influence on her and her feelings for Rito. And in order to end her rampage and bring her back to normal, Rito must stimulate her by attacking her with all of his perverted powers. After Mea, Nana and Momo join together to create an opening, Rito desperately suppresses his usual character and begins his shameless assault on Yami, groping and molesting her relentlessly until she is overwhelmed with stimulation. Yami finally returns to normal.
After things soon return to normal, Mea announces to Momo that she will help with the Harem Plan, much to Momo's shock and suspicion. The ever perverted Mea wants to experience the perverted powers that Rito displayed that managed to take down Yami. But also, she believes that Yami and Nana both love Rito, and decides to help them become more true to their feelings.
After the Darkness event, Mea notices that Yami is more emotionally reactive than usual, especially in regards to Rito, whom she still angrily denies having any feelings for. Mea tells Yami that Rito is tasked to tend to the schoolyard flowers, and suggest that Yami helps him to become better friends with him, which she initially refuses to do, but reluctantly decides to try later.