Kyoukai no Kanata: Shinonome Beyond the Boundary: Daybreak
Released on the seventh Blu-ray & DVD volume of the TV anime
Episode 0 takes place two years prior to the TV series when the Nase siblings meet Akito for the first time in a mission for Hiroomi.
Mitsuki follows her brother on this mission and their goal is to subjugate Akito.
[Source: MyAnimeList News]