SCHOOL DAYS: MAGICAL HEART☆KOKORO-CHAN

Originally an April's Fool in 2007, this one episode OVA is a spin-off of the School Days game by 0verflow, and shows Kokoro Katsura, the little sister of one of the main characters, from her other side. A mahou shoujo out to save the world, or something like that. (Source: AniDB) — Description from AniList community (anilist.co).