Jello is the pet of Runt Ironfurnace, he's one of his pets besides Fluffles. He is very small, appearing even smaller than a tiny slime. Runt initially found him when creating a box in the side of his house in the attempt to spawn a monster to increase his combat skill. Instead, he spawned a slime.
Jello has been seen to have ill reactions to sugary foods, such as when Stump fed him a slice of cake. He can also eat glowstone and turn it into fermented glowstone, which can be used to make a rocket potion.
Jello also apparently has a mysterious status effect that could only be noticed through Runt's Analyze Monster ability.
Aside from being a pet, Jello has been seen assisting Runt by distracting Urkk Doomwhip as he fought with Runt, turning the tide of the battle. Once Brio had questioned Runt about him, he said nothing because he felt that Jello had a purpose and that wasn't being a spy.
Kolb has agreed to feed Jello in the time that Runt left for Owl's Reach. He is mentioned in a later book by Runt after the village has been destroyed, as having been taken to the mines with Runt's parents.