Gideon Crane

Gideon Crane was a human whose mind temporarily fused with the Eighth Doctor's.

Shortly before Crane "[lost] his marbles," his body was occupied by William Bishop. (PROSE: Rise of the Dominator)

In 2003, Crane's mind fused with the Doctor's, including his memories. He then believed himself to be the Doctor and questioned the real Doctor about his experiences, believing that he was the Doctor as he had more of the Doctor's personal memories. The true Doctor, now known as 'Zebediah Doe', eventually determined that he was the true Doctor when his psychic matrix was dispersed among other inmates by a machine designed to extract mental imprints, as all of the inmates subsequently believed themselves to be the Doctor. Once the Doctor's mind was restored, it was revealed that Gideon Crane was actually a journalist who was sent from the United Kingdom to write about the creation of the state of Malebolgia. He retained enough of the Doctor's memories to disable the equipment while the Doctor dealt with the real threat. (AUDIO: Minuet in Hell)