Japanese voice: Tomoko Kawakami
English voice: Luci Christian
Mariko was the most powerful of all Diclonii, possessing 26 vectors (all of which were visible) with a range of 11 metres. She had little control over her emotions, as is common for Diclonii, and would swing between childish glee and homicidal rage at the drop of a hat. In the eleventh episode "Complication", she was released from confinement so that she could capture Lucy and exterminate Nana. To keep her under control, the Institute had implanted several micro-explosives within her body, one of which they were forced to set off shortly after releasing her when she killed Saito, her regular handler.
Shirakawa and Isobe lead the mission to find and put down Nana, bringing a Special Assault Team platoon with them for additional security. Nana, who at the time was becoming accustomed to living in Kaede House with Kouta, Yuka, Nyu and Mayu, could sense Mariko approaching, knowing that she was coming for her. Not wanting to risk the lives of her new family, Nana left the house and confronted Mariko at the beachfront.
Mariko took a twisted delight in battling Nana, seeing fighting and killing as merely a game and prolonged her amusement by making Nana suffer instead of killing her outright. Things become more complicated when Kouta shows up, apparently having followed Nana down to the bridge where the fight takes place. Kouta is deeply confused by the scene before him and shocked at the sight of the amputated Nana pulling her false limbs back onto her body. Mariko is infuriated at the prospect of Nana actually having friends and lashes out at her more aggressively than before, but Nana is able to briefly incapacitate Mariko by passing a vector through the little girl's head, which seems to "shut off" her vectors. With Mariko's vectors disabled, she is withdrawn from combat by Isobe.
In the final episode "No Return", Mariko recovers and once more battles Nana and also Lucy. While resting, she overhears Isobe musing over the fact that she is the daughter of Kurama. Mariko has never seen or heard Kurama before, but unbeknownst to her, she soon will.
As Mariko battles against Lucy on the boardwalk near the Kamakura-Enoshima bridge, the fight is interrupted by the appearance of Nana and Kurama. Realising that the man before her is her father, Mariko staggers toward him until he points a gun at her head. Mariko is confused by this and Nana warns Kurama of how dangerous she is, referring to him as "Papa" (Kurama and Nana had formed an attachment to one another over the years at the Institute). Mariko is outraged that her father treats Nana as his child and not her and she furiously lashes out at Nana. Kurama then drops his gun and walks over to Mariko, who cannot bring herself to raise a vector to him. Kurama embraces his true daughter, apologising profusely for all the things that he denied her in her short life. He confesses his sinful actions, how he had euthanised the Diclonius children of so many parents and selfishly spared his own child, only for her to endure a harsh environment of cruelty and suffering.
Holding Mariko in his arms, Kurama promises her that they will be together forever from then onward. He tells Isobe to let Mariko's timed explosives detonate and tells Nana to be good and find a happy life for herself. He then takes Mariko down to the bridge and they embrace each other tearfully up until the moment the bomb within Mariko explodes, killing them both.