Flame Core

The Flame Core is a Pureblood Heartless that was introduced in Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-.

Kingdom Hearts III
Flame Cores rained from the sky in Olympus during Hades's attack on the Realm of the Gods, crashing into buildings and lighting areas on fire. Multiple groups trap civilians, whom Sora, Donald and Goofy help rescue.

Design
The Flame Core's body is primarily spherical, similar to the Darkball, and has a large jagged crimson-colored mouth that makes up its upper body and lower head. It has a set of pointed wings, short pointed legs, and two beady yellow eyes above its mouth. On the top of its head is a pair of fiery horns which bend upwards, with two smaller extrusions from the sides.

Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage
Attacking it with melee attacks works to wear down its HP, but upon losing 50% of its HP, it lights itself completely ablaze and charges around, damaging Aqua. Three shots of Blizzaga can slay a Flame Core without much trouble.

Kingdom Hearts III
Flame Cores behave similarly to 0.2. Using Water or Blizzard spells in combos will prevent them from setting themselves ablaze, meaning Sora can finish them without issue. They can also be used to put them out while charging around the area. Crystal Snow's Formchanges, the Blizzard Claws and Blizzard Blades, exclusively deal Blizzard damage and can keep them doused as well. Water attacks will also cause the Flame Core to release steam and be unable to make any other action for 3 seconds.