|Portrayed by:||William Sanderson|
|First appearance:||Heart of Steel Part I|
|Last appearance:||Deep Freeze|
|Appears in:||4 Episodes|
|Karl Rossum Gallery|
Bruce Wayne once said that Rossum had taught him everything he knew about electronics, suggesting that Rossum was once a mentor to the young businessman. He also briefly served as one of theme park tycoon Grant Walker's "Visioneers". Rossum went into semi-retirement after his daughter was killed in a car accident. In his grief, Rossum thought of making a "perfect" world populated by androids instead of humans, where such things would never happen. To that end, Rossum resumed active leadership of Cybertron and designed and built H.A.R.D.A.C., a prototype artificial intelligence capable of producing androids that could impersonate living people.
Midway through his scheme's execution, Rossum realized that what he was doing was wrong, and that H.A.R.D.A.C. had gone beyond his control. He tried to shut H.A.R.D.A.C. down, but was incapacitated and imprisoned along with James Gordon, Hamilton Hill, and Harvey Bullock. They were rescued by Batman and Barbara Gordon, and he helped them all to escape Cybertron's building before an explosion that destroyed H.A.R.D.A.C. Outside, Hill assured him that he would not be held responsible for his creation's actions.
Rossum retired to a farm outside Gotham, settling for a more pastoral life, though still attended by a host of his robotic creations.
Unfortunately, his past came back to haunt him when a barely functioning H.A.R.D.A.C. managed to create a Batman duplicate. Although he ultimately survived, Rossum's greenhouse was destroyed in the fight between Batman and his duplicate.
Batman later tracked Rossum down when he suspected his involvement in Mr. Freeze's breakout. Rossum denied any involvement, but he did recognize the robot that took Freeze as being of a similar design to one he supplied to his old boss, Grant Walker.