Water Core

The Water Core is a Heartless that appears in Kingdom Hearts 0.2 Birth by Sleep -A fragmentary passage-.

Strategy
As it is a water elemental, Fire does not work on it. It can be electrocuted with Thunder and frozen with Blizzard, but is takes a few shots of either of the two to slay a Water Core.

Kingdom Hearts III
In the E3 2015 trailer, Water Cores attack Sora, Donald and Goofy in Mt. Olympus.