Kurt Flachfeld is Minna's deceased lover.

Before meeting his demise he was supposed to study music with Minna. After Minna volunteers to go to the front lines, he too goes with her, helping out in the evacuation of Gallia. Before Minna leaves for Britannia, Kurt promises to meet up with her there after he and the other maintenance crew members were evacuated. Unfortunately, this promise was not kept. Before Kurt was able to evacuated, a Neuroi beam engulfs the base in a fireball, killing Kurt.

(Source: Strike Witches Wiki)