Renning

He was successor to the throne held by King Ramon, but along with the king, he was defeated in the Mad King's War. Instead of being killed however, Daein turned him to their own knight, by experimenting him with Izuka's feral elixir. He then became Bertram, one of the Four Riders of Daein. After Ike and the Crimean Army defeated him, Bastian realized Bertram was Renning. Renning was later healed back to his true form, after Reyson sang the Galdr of Rebirth. Instead of becoming king however, he helped his niece govern Crimea.