Stupidity ( 間抜け / まぬけ / manuke / Stupid/Idiot ) is the ninety-first chapter of the Elfen Lied manga series.
As they seek Lucy, Nana tells Kurama that she knows someone who will be useful in opposing her, so they agree to meet at the beach in two hours while she finds that person, who turns out to be Kouta, whom she does not find in his hospital bed.
In the underground grotto, Chief Kakuzawa introduces Lucy to the world's only known Male Diclonius, who is his son from her mother. He declares his intent not to let what he believes to be the bloodline dilution of times past repeat itself by intermarriage with normal humans, so his son will mate with only another pure-blood Diclonius. Since the boy will be the new "Adam," Kakuzawa, as his father, will become the god of the new race. The boy, outfitted with the same mind-control device as the clones of Mariko, gingerly approaches her, recognizing the kinship between them. Lucy demands to know the fate of her mother. Kakuzawa reveals that she committed suicide not long after giving birth, her body dissected in an unsuccessful attempt to produce more pureblood Diclonii. He continues speaking at length on his notion and efforts to replace humans with Diclonii.
Anna tells her father that she lied to him about an important piece of information, which he interprets as about his impending 'godhood,' but he dismisses this as her not having known about her half-brother. He then states further that as she cannot predict her future, he will prove himself to be not as cruel a father as she thinks. He tries to reason with Lucy, calling their meeting a millennial event, and that they should unite, based on the mere fact that they have no place among humans, echoing the voice she has heard all these years.Taunted by the inner voice, combined with Kouta's vengeful vow after regaining his memories, Lucy, at last, remembers that one reason she had lived and endured all these long hard years, seeing the cap that he gave her when they were children. She tells him that she can make her world, but he responds that together, they can make the world for all their kind, by annihilating the 'foolish humans,' after which Anna tries again to tell him that he's missing a vital piece of information. Lucy beheads Kakuzawa and the boy, laughing over his corpse at the joke he has lived and mocking his speeches and grandiose notions, revealing that for all his hatred of Humans, he was one as well. The legends of the Original Diclonii were just that, and his people mistook a bone mutation for actually being Diclonii, which she would have sensed, had it been true. Lucy apologizes only to the corpse of her half-brother, saying that because their bloodline was causing the kind of misery and suffering that she always hated, even if for humans, it had to end, as it will soon with her death. She flatly asks Anna if she knew her family was human, and she confirms this, but also swears vengeance for her father's death, finally driving Lucy to split her massive head in two.
As a bloody tidal wave gushes, Lucy notes something the reader cannot see and then remarks that maybe Kakuzawa wasn't a cruel father after all.
- Yuka again harkens back to her school days, this time in a gym uniform, on the chapter's cover.