Conscience and soul that derives its form from fire and heat energy.
Lucinda, a god and one of the Underworld's rulers, was the first fire elemental. After her and her siblings were killed, Sparks -- her daughter -- is how techinally the Underworld's ruler via heredity.
I personally consider Fire Elementals to be the poor little meow-meows out of all of the kinds of Elementals, but that's a misconception. They're only incredibly fragile and easy to get snuffed out from water, too much wind, too little wind, or being "squished" when they're young and not yet developed or have consumed that much souls. They get more hearty with age and more fuel, and can reach a point in solidity and development where they could survive a rainstorm. The hotter and brighter they choose to burn, the quicker they go through their own energy and more often they have to seek out more food. Some choose to instead take on an ashy form that burns dimly from within and only feels lukewarm to the touch.