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. (AG: The Hexford Invasion, Survivors in Space)
Traits
Much like his the real Second Doctor, this one was well known to play the recorder. He also enjoyed stating "Oh my giddy aunt" whenever in shock.