Folken grew up in his kingdom, Fanelia, and was very close to his younger brother, Van, who was ten years younger than he. At the age of fifteen, Folken fought a dragon to become king as the Rite of Succession. Unfortunately, things did not go his way, and he lost his right arm in the fight. Because of how severe the wound was and because of how much blood he had lost, he naturally thought he would die. Emperor Dornkirk of Zaibach saved him though, and he was given a cybernetic arm, which, in my opinion, makes him a little unconfortable in his years of serving Dornkirk.
When his two catwomen, Naria and Eriya, die for Zaibach's cause, Folken realizes that killing to further Dornkirk's intentions is wrong. He abandons Zaibach and flees to Fanelia. Van learns of Folken's new location and flees to meet him. Once there, he learns of what happened to Folken that fateful day of fighting the dragon. Folken is not held on entirely good terms with Van, and Folken knows this.
Finally, in episode 25, Folken goes to meet and kill Dornkirk much to Hitomi's dismay due to a vision of him dying. He slays the Emperor but, sadly, he dies as well.