Gustav is Vincent Bari's partner. Bari and Gustav's relationship resembles that of a strict father and a rebellious son but Bari appears to (somewhat grudgingly) respect Gustav for his immense knowledge.
Despite often acting as a disapproving father, Gustav praises Bari when Bari sacrifices himself to save Gash and the others, saying that even though Bari had not become the king of the mamodo world, he had become a man that is powerful enough to even strike the king. He cries silently and bids farewell to Bari, which causes him to cry as well, promising that he will never forget him.
Gustav appears to have returned home after Bari's book was burnt and was shown to be praying hard for Zatch's victory against Clear.
