Gustave is a character in Shin Megami Tensei V.
Gustave is the keeper of the Cadaver's Hollow and the boss of the Miman. He offers items for purchase, adding Essences as part of his stock at a later point, as well as providing a healing service for a fee at leyline founts, becoming more expensive the lower the protagonist or his demons' health and stamina is.
He is also an avid collector, and will buy any Relics, human-era items from the ruined Tokyo landscape, from the protagonist, for Macca.
He takes an interest in the protagonist when he stumbles into the Cadaver's Hollow on accident while accessing the first leyline he had come across. He then offers his services to the protagonist and requests that he direct his wayward, airy, and forgetful Miman back to him should the protagonist find them in Da'at, offering successive rewards for every 5 Miman found.