Prior to the events of the story, Reisen worked on magitechnology for the Midgards military and invented the Mana Cannon. One day, Dhaos visited Reisen and warned him of the dangers of magitechnology, but Reisen refused to believe him and instead continued his research. During the story's past timeline, Reisen is the leader of Midgards's army. When Midgards is on the threshold of war with Dhaos's army, Reisen uses this opportunity to see what magitechnology is truly capable of, tempted to use the Mana Cannon only once Dhaos's army elevates the ground battle into an aerial one.
Reisen releases the power of the Mana Cannon, eliminating a large portion of Dhaos's army but consuming a large amount of mana in the process, which Martel herself feels. Reisen attempts to fire the Mana Cannon again, but it defects and explodes. Although Reisen survives the war, his whereabouts thereafter are unknown, as he is encountered in neither the present nor future timelines.