Unknown

The Unknown, also known as the Mysterious Figure, is a mysterious character whose identity is yet to be confirmed. The Unknown can be fought as a secret boss in Kingdom Hearts by Sleep and Kingdom Hearts: Light by Darkness.

Kingdom Hearts: Light by Darkness
''A cloaked figure whose identity is unknown. The person's purpose and origins are unclear.''

Kingdom Hearts: Light by Darkness
After the main storyline has finished, Sora, Riku, and Kairi travel to newly located ominous worlds, being the End of the World, The World That Never Was, and the Keyblade Graveyard, respectively. There, they will encounter the Unknown, and engage in battle with them. Afterward, the Unknown fades away, walking towards the opposite direction.