The Burnish are a race of mutant humans capable of freeform manipulation of fire. Due to the destructive, and some times uncontrollable, nature of the Burnish's abilities they are widely ostracized and persecuted by mainstream society.
During the events of the film, the Burnish are primarily in hiding after years of deliberate and systematic oppression by government-sponsored groups like the Freeze Force. The majority of Burnish activity seen by commonfolk comes in the form of attacks by a terrorist organization known as Mad Burnish. The remainder of the Burnish people elected to hide themselves away in areas close to high thermal activity in order to escape the thermal sensors used to track them.
Burnish, being originally human, appear almost completely identical to other non-Burnish humans. However, all Burnish are bonded to a portion of the Promare: this allows them to take control of and manipulate all sources of fire, as well as creating flames from their own internal flame. This symbiosis also makes Burnish nearly invulnerable to any physical damage as well as capable of regenerating their bodies should they suffer any wounds.
However, despite their increased resistance to damage, the Burnish are by no means immortal. Should a Burnish's flame be extinguished through any means, natural death or having their energy completely drained, the Burnish's body will begin to turn to ash before they eventually completely disintegrate.
A technique does exist to stave off premature Burnish death: akin to Cardiopulmonary Resuscitation (CPR), a Burnish can forcibly rekindle an expiring individual's internal flame by breathing their own flame into the subject.