The Earth Core is a Pureblood Heartless that appears in Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage- and Kingdom Hearts III.
Earth Cores appear as a Heartless resembling a hedgehog. Its body is round and oval shaped, with a pair of beady yellow eyes and a large purple jagged mouth as its face. It has several cracks in its body where its purple interior can be seen. Outside of its face, its entire body is covered with rocky spikes that extend outward. It has no limbs, instead using the spikes on its bottom to bounce around and swim through the ground.
Kingdom Hearts III
The Earth Core does not generate earthy matter for its attacks. Its face is the only vulnerable spot on its body. Any attempts to attack elsewhere with Physical, Dark, or Neutral attacks are simply repelled unless the player uses a Formchange with the Guardbreaker ability; collision with this Heartless briefly stuns whoever it hits, leaving them vulnerable for additional damage. This can be avoided with a well-timed Guard, by dodging, or having Stun Protection active. Their Drill Attack will also break Sora's guard and trigger Risk Dodge. While using Drill Attack or Spin Liner, Earth Cores can only be damaged by Fire, Blizzard, and Thunder attacks.
In Kingdom Hearts III, Earth Cores have the ability to travel underground. While this leaves them invulnerable to physical attacks in this state, they are still vulnerable to magic and non-Physical type attacks. They can also transform into Towers, where they have more health and can spit boulders and charge at Sora. However, if Sora uses Goofy Bombardier (which has a 100% chance for Goofy to activate while an Earth Core Tower is present) while locked on to a Tower, Goofy will go inside the tower and destroy it instantly. While visiting Kingdom of Corona, Earth Cores will never transform into towers.