Path of Radiance
Renning was the younger brother of King Ramon of Crimea. Prior to the birth his niece Elincia, he was appointed as heir to the Crimean throne; a position he retained after the birth of Elincia to prevent a blood-feud in Crimea. Renning lived his life as a soldier, he became leader of the royal guard and taught younger knights such Geoffrey and Kieran about how to become proper Crimean knights.
When the Mad King Ashnard invaded Crimea, Renning commanded the Crimean knights against the Daein army. Most knew that the Daein army had surprise and numbers on their side, but hope remained that Renning and his knights would overcome their foe. The last report of Renning's whereabouts came from the Gallian soldier Ranulf, he claimed that Renning had been defeated, and that Elincia was the last remaining Crimean noble.
The truth of Duke Renning's disappearance was much more complicated than a mere defeat in battle. After Elincia had left Crimea, Renning continued to resist against Ashnard. Ashnard was a man who respected strength above all else, and took interest in Renning. He ordered his men to kidnap Renning, and fed him Izuka's potion meant for creating feral laguz. Renning's strength greatly increased, but the potion drove him insane in the process. Ashnard made him one of the Four Riders of Daein and gave him the new name, Bertram.
Once Ike and his liberation army returned to Crimea, Bertram was ordered to defend Fort Pinell and to defeat the Crimean army. In battle, he crossed swords with Elincia, and immediately recognized her. He tried to tell her to run away, but his words were muddled by the effect of the potion. When battling Ike, he asked Ike to kill him so that he could end this nightmare, Ike questioned his sanity and defeated Bertram in battle.
Duke Bastian had recognized that Bertram was Renning while on the battlefield. After the battle, he arranged for Renning to be taken into custody secretly at Fayre Castle, so not to alarm Elinica. After the war had ended, he hired his long time associate Volke, and ordered him to find Izuka, to find a cure for the rapidly deteriorating Renning. Izuka offered to create an antidote in exchange for his freedom, but Bastian and Volke naturally knew that he couldn't be trusted.
When the goddess Ashera passed her judgment to the world, Bastian and Volke took Renning to Begnion in the hopes of finding a heron prince to use the Galdr of Rebirth on him. Bastian revealed the truth about Renning to Queen Elincia, King Tibarn of Phoenicis, and Prince Reyson of Serenes. Reyson agreed to perform to galdr, and successfully cured Renning of his condition. At the Tower of Guidance, Renning obtained his armor and trusty steed, joining Elincia in battle against the forces of order. After the battle, Renning did not pursue the Crimean throne, and instead supported Elincia in rebuilding the kingdom.