Kyoukai no Kanata: Shinonome (Beyond the Boundary: Daybreak)
Bundled with BD/DVD 7.
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]