Fate/kaleid liner Prisma Illya: Vow in the Snow

Shirou Emiya, against his adoptive father's advice, treats Miyu, a girl found in a ruined city, as family. Miyu begins attending school, but the Ainsworth family, along with their agents, see her as a weapon and kidnap her with the intention of sacrificing her. Shirou is forced to confront the reality of Miyu's immense power, which is capable of both great good and devastating destruction. He must now embark on a rescue mission, battling to save his adopted sister from the Ainsworth family. Simultaneously, Shirou must grapple with the inherent danger that Miyu's abilities present, forcing him to consider the implications of her power and its potential consequences.