Born in the pre-federal Belkan state, Dietrich Kellerman joined the Belkan Air Force in 1973 among a general reorganization and the ongoing territorial disputes between Belka and the eastern countries of Osea, being assigned as the leader of the 51st Air Division's Silber Squadron. During the Rectan War, Kellerman used his leadership skills in direct action to break through the Rectan air superiority over Mount Mainz and allow ground forces to besiege Calle City, earning himself the alias of "Hero of Mainz" and becoming the top ace of the BAF.
Following the defeat of Recta and birth of the Belkan Federation, Kellermann continued to serve as a Lieutenant General with the first-generation Silber team, which achieved great results in exercises with aggressor units, before retiring from the Air Force in 1990. Driven by the opinion of his superior, Major General Heinreich Land, he became a military flight instructor in charge of his own school, the 9th Special Education Course or "Kellerman Institute", which birthed various elite pilots such as Michael Heimeroth.
After the outbreak of the Belkan War, Kellerman was forced to rejoin the Silber Squadron and return to the frontlines to bolster the morale of friendly pilots after the Allied Forces' invasion of South Belka. Many pilots from the second-generation Silber Team that existed since his retirement had either died or become too injured to fly, leading him to command former pupils from the Institution as a replacement.
On May 28 of 1995, Kellerman and his team were dispatched to back Belkan forces in the historic Operation Battle-Axe in Area B7R, who had recently suffered heavy losses on account of the mercenary Galm Team's intervention. He and his men were engaged and shot down by Larry Foulke and Cipher, whom he recognized as understanding the laws of aerial combat. The battle saw the death of the entire squadron except for Kellerman himself, who survived the encounter.
Burdened by grief over the loss of his students, he decided to permanently retire from the Air Force. He was later tried as a war criminal by the Osean Federation on falsified charges in an attempt to hire him as a flight instructor, but refused the offer and settled down as a farmer in Birenheim, Belka.
- "He was good. From what I'd heard about him, I thought he was still just a young mercenary. But he'd matured since then. He still wasn't perfect, but I could see he had come a long way toward understanding the rules of combat. What's important on the battlefield is to let go of hate, to survive, and to adhere to the rules you've set for yourself. These are the rules I've taught my students. And when I saw him, I could tell my time was done. A new generation had inherited the sky. There was no more need for an old soldier like me."
- ― Kellerman describe about Cipher
- Kellerman's experience of being tried by Osea on false charges was likely inspired by a similar life experience of Erich Hartmann, a WWII pilot that partly inspired Yellow 13.