I think i get it now.
The spirit of the blanket possessed this girl. The girl knows all about the MC because of the blanket or they know each other when they were young.
Maybe the girl liked the MC back then but the MC was more into his blanket and kinda ignored her.
Anyway, the girl is influenced by the blanket, that's why she and the blanket swap soul, or the body of the girl transforms into a blanket. Main reason is the blanket is jealous of how the MC focuses on the girl now instead of the blanket.