The Second Doctor was a clone created from the Fourth Doctor. The clone, however, appeared as a copy of the Doctor's second incarnation. He had the Doctor's real memories, and indeed thought he was the Doctor. The Fourth Doctor met him and proved his false identity, but not before Hexford was moved into space and the Shiscari infant nearly devoured him. The Fourth Doctor said that due to his cloning type, he would not live more than a few months. (AUDIO: The Hexford Invasion, Survivors in Space)
Personality
Much like the real Second Doctor, this one played the recorder. He also enjoyed stating "Oh, my giddy aunt" whenever in shock. He had a working Sonic screwdriver.
However, this Doctor was aggressive and untrustworthy. In an early encounter with Fenella Wibbsey, he yelled at her, an action unlike his normal incarnations. He betrayed the Fourth Doctor, Mike Yates and the town of Hexford. He also cursed Mike Yates for his slow wit. (AUDIO: The Hexford Invasion, Survivors in Space) The Fourth Doctor called him misguided rather than evil and allowed him to remain at Hexford after it was returned. (AUDIO: Survivors in Space)