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.
He wanted to kill the slime, but realizing it wasn't hostile, he spared it, clearing out a double chest for the slime. He named it Jello, an Earth food talked about by Steve.
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 potion of leaping ll also known as a rocket potion.
Jello also apparently has a mysterious status effect.
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.
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.