Golem:
(Chapter 8)
These Halfhearted are the weakest of all Halfhearted, but they're still no slouches. They are vaguely humanoid, but don't have a definite form, so they can ooze through solid surfaces and attach to anything. Once they have a hold of something, they don't let go. They can also reroute spells, taking in supportive spells and deflecting negative spells. Their one weakness is their indefinite form. A Blizzaga will freeze them solid, making it easy to smash them to pieces.

Medusa:
(Chapter 11)
Medusas have a more defined form than Golems, giving them less maneuverability and also removing the ability to reroute spells. Talk about your bad hair days, though. They have long fingers and dreadlocks that act like whips, leaving painful welts on anything they touch. Medusas can also wrap these appendages around things. When the appendages are severed, they lose form and cling to anything that touches them. Their form is still indefinite enough to make them weak against Blizzaga spells.

Dark Chimera:
(Chapter 22)
These nasties are one of the stronger Halfhearted, giving them a more defined form. They have an assortment of parts from animals of the savannah. Most of their body resembles a baboon, but their limbs end in lion's paws and the paws end in lion's claws, which should be avoided. They also have gazelle horns on their heads and tend to use these to charge. They charge with the horns pointing up for more of a headbutt when they want to ooze up the opponents. While oozing, they become like Golems and can reroute spells. It's when the horns are pointed at the target that you need to start worrying. This means the Chimera is going to shove its horns in your side, which hurts...a lot....

Berserk Armor:
(Chapter 14)
As if Guard Armor and Opposite Armor weren't bad enough, this third form appears in front of the Keyhole in Traverse Town when the remains of some Berserker Nobodies combine with the heart that Opposite Armor releases. Berserk Armor is the Halfhearted version of Opposite Armor with two major differences, double claymores to bash your head in with. So yeah, this Halfhearted has a very well defined form. It is able to selectively lose form as well as redirect spells. These abilities must be consciously used, as opposed to Golems, which do them unconsciously. Unfortunately, Berserk Armor is too large for Blizzaga spells to work effectively. The shattered appendage can just regenerate. It is susceptible to intense vibrations.

Henry J. Waternoose:
(Chapter 17)
Waternoose put his whole heart into Monsters, Incorporated, literally. His determination to keep the company from going under, regardless of the consequences, turned him into a Heartless. With some Nobodies and dark magic from Astan, Waternoose transforms into a Halfhearted to do battle with Sora and company. The signature opening move of this particular Halfhearted is to Silence anyone in his way, making it impossible to cast magic spells, though Limits can still be used. He can also extend and retract his legs at will, making it hard to approach him for close-range attacks. His form is very defined and he has to concentrate deeply to make it indefinite, like the Golems. When stunned, he is open to physical attacks. The final blow is struck by the mischievous tike, Boo.