Hellflame is an extremely powerful Quirk that gives the user pyrokinetic abilities.
Enji is able to generate large fire blasts to incinerate his opponents, as well as cover his body in flames. Changing the temperature of the flames is possible as well, which means that their damage output is capable of increasing or decreasing depending of Enji's wishes. The anime adaptation shows that those flames can heat up to the point of turning blue.
Hellflame will raise Enji's body temperature to dangerous levels if overused due to successive powerful attacks or drawn-out battles, impairing his physical abilities.
Enji likes to keep his body in a flaming state, in order to look more imposing as the Flame Hero Endeavor.
Endeavor fights by using his flames to overwhelm his opponents with extreme heat. His attacks include large-scale flamethrowers, precise heat-rays, and fists of fire among other options.
Endeavor uses low-temperature flames on weaker opponents to be used as a warning shot that normally incapacitates Endeavor's targets, or to minimize the strain on his body. The Flame Hero raises the heat whenever he's trying to finish off the opponent; high-temperature attacks are meant to defeat foes definitively, as they raise Enji's body temperature significantly. This is a high-risk tactic that can backfire if he uses too many of these techniques.
Hellflame can also be used to prevent Endeavor from falling or suspend the Flame Hero in mid-air by expelling fire jets through the soles of his feet. He can also eject flames from his back or feet to propel himself forward through the air, essentially giving him the ability to fly. This same ability can be used as a reverse thrust should Endeavor need to resist an attack.
Endeavor's signature move is called "Flashfire". It raises his flames to their maximum temperature to create a powerful heat-ray that vaporizes any target. The heat from this attack raises Enji's body temperature to its limit and immediately impairs his abilities going forward.
The anime shows more combat applications for Hellflame. Endeavor can shape his flames to function like various weapons and is able to heat up the soles of his feet to anchor himself to walls through molten footprints, allowing him to run along their surface. Burning down a target at a cellular level is possible for him as well, letting him bypass healing or regenerative Quirks with flames of extreme temperatures.