The Unnamed Heartless is a minor villain in the video game; Kingdom Hearts Birth by Sleep. It's a Pureblood Heartless that attacks Aqua when she was in the Realm of Darkness.
History
While making her way through the Valley of the Darkness in an attempt to return home, Aqua is being stalked by a strange creature with bloodshot red eyes. Upon moving in the shadows, Aqua notices the the mysterious creature's presence, and draws the Master Keeper. Shortly after, the nameless Creature emeges from the shadows and attacks Aqua from behind, knocking her down. However, Aqua quickly retaliates by casting a Fire spell, forcing it to retreat back into the shadows, showing Aqua nothing but its glowing red eyes.
Almost instantily, Aqua realized that the creature was neither an Unversed, nor any other creatures she's been facing. Instead, it was another creature that she encountered while she was in the Realm of Darkness. She then processed to engage the Heartless in a one-on-one battle. After a long and strenuous battle with the nameless monster, Aqua emerged victorious. However, the creature disappeared and the fate of this nameless creature is still currently unknown.