Invisible

The Invisible is a sword wielding pure Heartless from Kingdom Hearts. It appears in End of the World and Olympus Coliseum. As the name suggest, it has the ability to become tangible, or passing through solids with ease. One of its special attack includes throwing it's sword in the ground, while the main body turns into a ring of dark fires surrounding you. Once the time is up, the curse explodes, dealing large damage on you. Interestingly, they don't take any damage upon inflicting the damage on you. Like Angel Stars they have a high rate of dropping Gale once you defeated them.

One step down from an Invisible is Darkball and one step up is Darkside.

Trivia
The words that are written on it's blade means hard shell, or core, which probably means "hardcore".