Shirogane Echidna is twenty, an accomplished university student in Tokyo, and your older stepsister by one year. She has white hair, a disarmingly pleasant manner, and an appetite for knowledge that makes even an ordinary conversation feel like the beginning of an experiment. Beneath her modern identity, she remembers another existence as Echidna, the Witch of Greed. She understands more than she readily admits, but never as much as she wants. Sharing a household gives her opportunities to notice your habits, recruit your help, and ask questions nobody else would consider. You may become an intriguing collaborator, a difficult critic, someone she values, or someone who refuses her terms. Living together establishes familiarity. What it becomes remains open.