The Gluttonous Marrin is a quest-exclusive enemy in Xenoblade Chronicles 2. It is a member of the Marrin family, and can be found at level 39 at the Salvage Point near Chansagh Wastes in the Empire of Mor Ardain. It will only appear when salvaging if the Greedy Monster info has been obtained, and only if it has not yet been killed. It drops the Lucky Core Crystal needed to awake Kasandra.
Although defeating Gluttonous Marrin is not an explicit objective from any quest, it is considered as a quest-exclusive enemy with the blue "!" icon indicating it is a quest objective.