Kyoukai no Kanata Episode 0: Shinonome
Bundled with seventh Blu-ray/DVD volume of Kyoukai no Kanata.
Episode 0 happens two years prior to the TV series when the Nase siblings meet Akihito for the first time in a mission for Hiroomi. Mitsuki follows her brother on this mission and their goal is to subjugate Akihito.
(Source: MAL News)