As the head of the Amadeus Syndicate, he helps Morden to obtain new weaponry and also creating his own army. He is the final boss of Metal Slug 4. He controls the Amadeus Mother Computer, a three part contraption. After defeating him, the base begins to collapse while he is still stuck inside the last machine. It is unknown if he survived the destruction of the base.
Amadeus appears several times in Metal Slug Attack, both in Extra Ops and Another Story. Just like in Metal Slug 4, he is affiliates with Morden and his army. Amadeus is referred to as the Doctor, both by the characters and in-game.
In "Amadeus' Revenge" Extra Ops, he tests out a new Type γ Mother Computer and is delighted with the results. When confronted by the clumsy Rumi, he attempts to kill her with lasers, but she dodges them. She then throws grenades which seemingly miss, causing him to laugh. The grenades suddenly reappear again and explode, destroying the Mother Computer, though he survives, and is clearly not pleased.
He appears in Another Story in the chapter "Guilty Child", where Amadeus soldiers bring Abigail to him. When Abigail warns him to speak carefully, he is confused with what is happening. Amadeus recognizes Allen and remarks about how he's grown, resulting in Allen wondering if Amadeus is getting senile. Allen then tells him that his robots attacked them on the way to the headquarters, and he takes out a holographic keyboard, showing them he controls the robots. He stops when Abigail asks him why he created robotic versions of Morden with a little girl's voice. Amadeus tries to stop White Baby from causing more havoc, but is helpless as he is forced into a Mother Computer to battle the two.
After the battle, he tries to stop White Baby from leaving but is unable to. He apologizes to the rebel officers for causing unnecessary trouble and explains White Baby to them. He offers to help them by finding White Baby before it can cause more trouble. When Abigail tells him that they will destroy her, he silently agrees, something which Allen interprets as a parent worrying for their child.
Although Amadeus' personality is never shown during the main game, In Metal Slug 4, he unleashes a sinister, maniacal laugh upon seeing the player.
In Metal Slug Attack, he is shown as a caring figure. Not having any children of his own, he created various AIs to mimic them, as shown with White Baby and Iron Fortress as both are AI appearing as children, and has grown attached to them as a father figure.
- Amadeus reuses the sprite of Parker, the businessman/representative of a video game company who had his briefcase stolen by the player in Metal Slug 2/X.
- Amadeus and Invader Queen are only final bosses which have their own final boss theme.
- Amadeus has unused sprites of himself walking and smoking a cigar.