Subject X

Subject X is the name given to a mysterious girl mentioned in the Secret Reports of Kingdom Hearts III.

She was an amnesiac teenage girl found in Radiant Garden sometime when Terra-Xehanort was a researcher for Ansem the Wise. She was taken to a cell and investigated by researchers. All she could remember was four friends, and a key. She did not speak for a week until she was found. Her words imply she left her home world with others, but she could not recall who they are. She knows the saying "May your heart be your guiding key."

She enjoyed talking to two boys who would visit, who were Lea and Isa. They would infiltrate the castle and find a way to set her free, although it appeared she had been moved. Lea and Isa eventually decided to become apprentices of Ansem the Wise in order to find a lead on her.

After experimentation on her by Terra-Xehanort, Ansem the Wise felt that the research on her had gone too far, after she suffered a mental collapse, and he ordered her release and the research to be destroyed.

An eye-patched man, presumably Braig, came to take her from her prison.