Runty is Billy's pet Fusion Spawn. At first, many thought Billy simply yearned to have another pet, so he caught and tamed a Fusion Spawn. However, it is revealed that Runty is in fact Billy's Imaginary Friend—giving explanation as to why he was not under Fuse's control.
When the player infiltrates Fuse's lair in an attempt to weaken its defense shields, Billy shows up with Runty; offering assistance and Runty is sacrificed—giving all of his Imaginary Energy to lower the shields to Fuse's Labyrinth.
- A health bar appears for him like all other enemies, although he cannot be attacked.
- Though Runty is supposedly destroyed, he still appears next to Billy in Endsville.
- He is the only Fusion Spawn in the game to be able to talk (saying "Mama?" when the player comes near him). However, this cannot be counted because Runty is an imaginary friend.