Karl Heiser, a seemingly jolly, friendly man, was a member of the Waffen S.S. during World War II. He served under S.S. officer Conrad Stauffer and was friends with Stauffer's mistress Maria Sorenson. He was secretly in love with Maria, constantly spoiling her with gifts, including her own personal S.S. bodyguard, Fritz.
Beneath his jolly exterior, Heiser was a very ambitious man. In addition to coveting Stauffer's girlfriend he also wanted Stauffer's job, and was obsessed with the idea of succeeding him. When Stauffer went to American with Baron Ikito in 1941 to retrieve Jack Griffin's invisibility formula from his grandson Frank Griffin, Heiser immediately took up residence in Stauffer's office after rumors began to spread that Stauffer had been killed.
However, Stauffer was very much alive, and upon returning to Germany a year later in 1942 immediately had Heiser arrested and sentenced to death. Frank Griffin, now invisible after taking his grandfather's formula, helped Heiser escape from prison in return for Heiser telling him where in America some German bombers would strike. Ultimately, Heiser betrayed Frank by informing Stauffer when he and Maria were taken to the Japanese embassy in Berlin by Ikito.
Heiser mistakenly believed that this would allow him to get back on Stauffer's good side, but instead Stauffer sent two S.S. men to find Heiser and kill him. Not knowing this, Heiser went to the Japanese embassy in time to witness Stauffer's death at the hands of Ikito, and, overjoyed at his new promotion, pursued Frank and Maria to the airfield to stop them from escaping, only to be shot and killed himself when Stauffer's S.S. guards finally caught up with him.