The Ghost in the Closet is a mysterious ghost girl who lives in Max's closet.

Despite being visible to the viewer, few individuals in the realm are able to actually see her. Neither Max, Maou nor Zenia can sense her, but Fred could, causing him to flee Max's apartment in fear when they first reunited after Maou's defeat. Even tho Maou is unable to see her she is able to perceive her on occasion, sometimes Maou hers what she is talking...

(Source: Level 1 Demon Lord and One Room Hero Wiki)