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The Orb of Light is not a true antagonistic enemy, but instead is part of Terra and Aqua's Mark of Mastery Exam. The first stage of their exam consists of battling several of these, which Master Xehanort has tainted with darkness. Ventus then rushes to Terra and Aqua's aid. The Orbs of Light are defeated with ease, and Terra and Aqua continue their exam.
The Orb of Light was originally a simple, white orb that gives off a yellow radiance. When Master Xehanort corrupts the orb with darkness, it gains a layer of fluid-like Darkness but retains its original yellow radiance and white core.
The Orb of Light is shown to be able to move around, attacking with several beams of light.