The following page contains spoilers that can reveal information about said subject that you, the reader, may not want to discover. It is highly recommended that you carefully read the following subject or risk exposing yourself to knowledge or information that you may not want to read.
Bertram was one of Daein's previous riders. He was defeated by Crimea in the Mad King's War. During part four, however, it is revealed that he is actually Elincia's uncle Renning, who was believed to have been killed in the Mad King's War. He had been drugged with the drug that creates Feral Ones, and the drug had warped his mind. Bastian had actually left on a trip to research something after the Mad King's War, and he had left to research if there was a way to cure Renning from the drug. He hadn't told Elincia that Renning was still alive, because he didn't want to get her hopes up. Renning becomes a playable character for the first time at part four's endgame.
Related ThreadsIs Bertram Renning? (Spoilers) - last post by @ Mar 13, 2007
Bertram = Renning? (SPOILERS) - last post @ Dec 29, 2008