Goo refers to both a sentient blob that Tedd accidentally created in science class (it is unknown how he managed to create the thing out of materials found in a high school) and a more dangerous version of the same created by Lord Tedd, apparently sent to kill the Main Universe's Tedd and Elliot.
(Note: it is never explicitly stated, but presumed, that details about the composition of the two Goos are the same for both.)
The Goo is "a delicate balance of liquids and solids" according to Tedd, and even slightly upsetting this balance could cause the Goo to die. The "goo" part of the Goo is actually the outer armour of a demon nuclei, the marble-sized sentient part of the monster. The outer shell that gives the demon nucleus the "Goo" nickname is simply materials scavenged up to form a protective covering.
In the first story arc, Tedd and Elliot's science class were all being taught how to make "goo"; however, Tedd and Elliot's somehow came to life and escaped (presumably, a demon nuclei found its way into the substance during its creation). It evaded capture and ate a box of steroids, causing it to grow to immense size and attack its creators. After Tedd's plan to saturate the Goo goes awry because the sprinkler system doesn't work, Elliot sets fire to the Goo, presumably killing it.
The Demon Nucleus from the first goo was revealed to have escaped the explosion and developed a second, larger "shell" underneath the school. It is then revealed that the Omega Goo was sent by Lord Tedd to kill the Main Universe characters. It was larger than it was originally; having grown claws, mouths, and horns, and even gaining the ability to shoot fire. Ellen destroyed the Omega Goo using Tamashii Gekido after a lengthy fight involving Nanase, Susan and Sensei Greg. Ellen then stepped on the demon nuclei, destroying it.
According to a computer screen in Lord Tedd's home dimension, the Omega Goo is designated as "Goo Cell #0012", and has apparently failed its objective.
- When zapped by a transformation beam, the Omega Goo is stunned instead of turning into a "slime woman".
- The first Goo was apparently afraid of the hero from Dragon Warrior.
- It looks like the lighter used to destroy the first Goo was borrowed from Silent Bob.