PENGUINDRUM (Manga) Vol. 1

Twins Kanba and Shoma live with their terminally ill little sister, Himari. One day, they visit the aquarium, a place full of warm memories for Himari, but she suddenly falls ill--and dies! At that very moment, a voice cries out, "Survival Tactic!" and Himari, wearing a penguin hat, comes back to life...?!