The entity is revealed to be Baalmon , a Digimon known as the "King of Knowledge" who holds a book containing all the world's information.
The episode reveals that the "ghost girls" assisting Baalmon were actually real ghosts , not Digimon, a rare occurrence for the series. Technical & File Context DigimonGhostGame_Ep_53_SUB_ITA.mp4
Every time Kiyoshiro loses a round, the entity "steals" his knowledge. As a result, he begins to forget his friends, his work, and eventually his own partner, Jellymon. The entity is revealed to be Baalmon ,