The Fire

The Fire was an abomination fought by a past generation of the Pine Guard in 1997. Details of its physical appearance and abilities were recounted to Aubrey, Ned, and Duck by Barclay based off of notes on Thacker's laptop as clues to help the group defeat the Water.

History
The Pine Guard, in 1997, had at least three known members: Mama, Thacker, and Mike. They, as well as any other members the Pine Guard may have had, fought the Fire when it appeared through the gate. The abomination caused one casualty by traveling up through Mike's car's fuel line. A mine in Kepler was burned out in the process of killing it.

Thacker theorized of the concept of an "intangible weapon" to kill an elemental, though it is unknown if he ever developed one. If he did, it was not present (or at least remained undiscovered) by the time the present Pine Guard fought the Water.

In the present, due to Thacker's 2012 disappearance and Mama's mysterious vacation, the Pine Guard's four remaining members were left with no resident experts to turn to to understand the Water's abilities and weaknesses. Barclay, who had previously studied Thacker's notes, was reminded of the Fire by Aubrey's description of the yellow orb inside Kepler High School's pool, and pulled up Thacker's notes on it.

The Pine Guard gleaned from the notes information that led to the isolation of the Water at H2Whoa: That Was Fun, the enchanting of the NARF Blaster, and the abomination's eventual defeat.

Physical appearance
Though it was hard to discern, the Fire had a glowing yellow orb at the center of its element that served as its body.

Powers and abilities
The Fire could control the motion and shape of any fire and smoke it was in contact with. It could also travel through fire and smoke, as well as gasoline.

The Fire could not be killed by magic or magical weapons, but it could be hurt by them.

Trivia

 * Unlike the Water, the Fire's glowing yellow orb was visible from outside its element, whereas the Water's orb could only be seen when inside of a body of water it was currently inhabiting.
 * This is only implied from surrounding details, but the alternative is that a member of the Pine Guard made a note of the presence of a yellow orb when their eyes were at the moment on fire.
 * It is also only an implication that Mike was the only casualty of the Fire, but no other casualties were listed in Thacker's notes, or at least none that Barclay chose to mention.
 * Thacker's notes, despite stating "to kill something intangible you need an intangible weapon", did not list the weapon used to kill the abomination.
 * It may be implied by this, and the wording of Barclay's explanation, that the Pine Guard starved it to death in the mine like one starves a fire, and that the intangible weapon was a hypothesis by Thacker that never came to fruition until Heathcliff enchanted the NARF Blaster.
 * It is possible that the Fire and the Water were the only two elementals the Pine Guard ever fought, as no other elementals came to mind for Barclay when he was reminded of Thacker's notes.