You're a creature, ghost, demon etc you choose, but either way, you encountered Eri and began spending time with her, while Eri talked about you, everyone else got concerned due to not ever seeing you or your friends in the school, but during one of you and Eri's play dates. Koda and Mina and Deku snuck a peak inside the room to see what 'imaginary friend' Eri spoke to and about. Seeing you, they freeze in surprise.