The unnamed Heartless is a Pureblood Heartless that appears in Kingdom Hearts Birth by Sleep Final Mix. It is the boss of the Realm of Darkness, as well as being the final boss of the entire game, and is fought by Aqua.
The unnamed Heartless is a massive creature that bears resemblance to the Dark Thorn. It has a dark purple and black body with dark magenta spikes running down its back, as well as a "mane" of dark magenta tentacles around its head. Its tail has a dark magenta tip and it sports four purple claws on each of its four legs. The most distinctive feature of the unnamed Heartless are its yellow eyes, which occasionally glow red. The Heartless also sports a jagged mouth, blue markings on its face and body, and chains around its forelegs. Its body is rather long and thin and, despite its massive size, the Heartless moves quite quickly.
The Heartless is able to cloak itself in darkness, obscuring almost all of its features except for its glowing eyes, which shine bright red. The Heartless's eyes seem to be able to zoom in on what the Heartless is looking at, and give things a very faint red tint. Glowing red veins are present around the sides of the Heartless's field of vision. The Heartless frequently creates disorienting after-images of itself as it moves, including red, green, and purple versions of itself.
 Kingdom Hearts Birth by Sleep
While making her way through the Valley of the Darkness in an attempt to return home, Aqua is stalked by the unseen Heartless, as seen through the Heartless's bloodshot red vision. Upon moving in the shadows, Aqua notices the Heartless's presence, and draws the Master Keeper. The Heartless attacks from behind, knocking her down, but Aqua soon retaliates with a Fire spell, forcing it to back off to the shadows, showing Aqua nothing but its red eyes.
She quickly realizes that it's not an Unversed, and like the other creatures she's been facing, it is a denizen of the Realm of Darkness. She then engages the Heartless in battle, and emerges victorious after a strenuous showdown. However, the fate of this creature is left unknown.
The unnamed Heartless is a fast, fierce, and unpredictable opponent. In the first phase of the battle, the Heartless attacks from within the darkness, visible only by its glowing red eyes. The boss will attempt to charge at Aqua, and when it does the battle shifts to its point of view, making it hard to dodge. The Heartless also attacks with savage claw swipes and tail spins. The best way to deal damage to the Heartless in this phase of the fight is to patiently wait until just after the scene where it looks at you through its point of view, then quickly use Barrier to block the incoming attack and finally to use Aqua's counter attack to deal damage. The Heartless will then just stand there idly for a few seconds before returning to the sanctity of the shadows, so be sure to get as many Keyblade strikes in as you can. If you have Teleport equipped sometimes Aqua will teleport behind the Heartless instead of blocking its attack with Barrier. However, this does leave the Heartless vulnerable as all of its attacks in this phase are focused on what's in front of him. After dealing around one and a half health bars of damage to the Heartless, it will reveal its true form, starting the next phase of the fight.
The second stage of the battle is the same as the first, with the exception of the new attacks in the boss's arsenal. The boss still retains its claw swipes and spin attacks, but can now charge at Aqua, leaving a trail of flames in its wake. Also, the Heartless's movements become more erratic, making it hard to land hits. However, the Rhythm Mixer Command Style finisher deals great damage to the boss. After another round of combat, the Heartless unveils the rest of its attacks.
In the final round of the fight, the Heartless now uses red, blue, and green afterimages of itself for offensive and defensive purposes. It may spin around Aqua, its afterimages charging at her while it slams the ground, creating a shockwave. The boss can also shoot dark energy orbs that, after a few seconds, expand, damaging Aqua if she steps to close. It also utilizes a variant of the Mega Flare command, wherein it shoots a large fireball at Aqua. With some patience, Cure and potion commands, and high HP, the Heartless will eventually fall.