Given: Uragawa no Sonzai (うらがわの存在, Existence in the Other Side?)
is the OVA that tells an "other side" story centered on Mafuyu Sato and Ritsuka Uenoyama that wasn't featured in the movie.
The official website for the manga announced that the special edition version of the seventh volume would bundle an original video animation (OVA). It was first released on December 1, 2021.
Volume 7 OVA Special limited edition cover