Jeff is a giant spider who thinks Billy is his father. When Billy opens Grim's magical trunk one day, he finds an egg, which he incubates himself and raises. When it hatches, Jeff appears and believes Billy is his father. Unfailingly friendly, caring, and kind, Jeff wants more than anything to gain Billy's fatherly love despite Billy's arachnophobia-based fear and hatred for him. Billy frequently beats him with blunt objects. Jeff blames himself for Billy's hatred toward him and constantly tries to win his fatherly love and affection. With Mandy's help, he finally manages to scare Billy into loving him. Jeff is described by Mandy as being both "too nice" and "basically insane".
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