This is the OVA for Watashi ni Tenshi ga Maiorita!.
Synopsis[]
The girls go camping at the lake for their vacation, bringing a reluctant Miyako with them. Koko recalls her friendship with Miyako, through rose-tinted goggles, of course. During Halloween, Miyako anticipates what costume Hana will be wearing, only to end up having to cosplay herself for Koko and Yuu. Miyako recalls when Hinata was born, quickly bonding as sisters.