Fat Man is an obese man who at the age of 10 built an atomic bomb by using a handbook from the internet. He planted bombs on the Big Shell in Metal Gear Solid 2, and was the first boss fight in the plant chapter. He uses roller skates to move quickly, and wears an extremely heavy protective suit which is nearly impenetrable to gunfire.

He first joined NAVSCOLEO, then was kicked out and was taken in by NEST, but left and joined with Dead-Cell, the terrorist group in the Plant Chapter of Metal Gear Solid 2. He was operating under the orders of the Patriots, an illumnati-esque group to test Raiden on his progress.