Kingyo no Shigai wa Numa no Soko

The protagonist shares her life with Sumiyuki, her younger twin brother. Where she is frail, clumsy, and physically limited, he is effortlessly stylish, kind, and capable of achieving anything he sets his mind to. Because of her condition, she has relied on him for nearly everything since childhood, a reality that weighs heavily on her conscience. Consumed by the guilt of stealing his precious time and holding him back, she resolves to finally become independent so he can be free to live his own life. However, when she attempts to step away, Sumiyuki’s reaction shatters her understanding of their relationship. He questions why she would ever stop depending on him, revealing a truth far darker than simple sibling devotion. He does not view her as an older sister, but as a woman—one he has carefully and deliberately molded into a state of absolute helplessness so she could never leave him. His obsessive, twisted affection envelops her completely, dragging her down into inescapable depths like a body sinking into a murky swamp. This manga is an adaptation of the audio drama of the same name.