It is unclear exactly who or what Grandpa Demetto was, as his origin was never explained. When CologneDuelsYuma Tsukumo, Grandpa Demetto appears as a part of her Field "Doll House", "fixing" Cologne's destroyed "Doll Monsters". She says that if Yuma "breaks" her dolls, Grandpa Demetto will fix them for her. When Yuma's friends make bad comments about Demetto's appearance, Cologne screams that Grandpa Demetto is always doing his best to fix the dolls, and that she won't allow them to treat him as a fool.
After Cologne lost her Duel against Yuma, she was thrown to inside the Doll House which was in fire by the attack of Yuma's "Number". Cathy tried to save her, but in her haste she mistakes one of Cologne's dolls for her instead. This time, Yuma decides to go save Cologne, but, who comes out of the Doll House carrying her in his arms is the one who gave Cologne a soul - Grandpa Demetto. Cologne's soul was supposed to leave her body, but Demetto gave her his own soul, which allowed her to keep alive. While he sends Cologne out of the fire towards Yuma, he is engulfed by the flames together with the field. Cologne mourns his death, saying that she loved him.
Demetto may be a reference to Gepetto from the Pinocchio fairy-tale, as he is a kind old craftsman, who created a doll and gave it a life.