Kurosaki Mea/Plot

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 recently 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 takes a liking to her, not caring she's a transformation weapon.

In Darkness Chapter 39, just when Mea  felt accepted by everyone and no longer see's herself a weapon, Yami's "Darkness Transformation" had activated along with the dark consciounce within Mea takeing hold, changing her appearance and personality.