Dr. Doppler was the first Reploid scientist ever created and was well-known amongst the populace for his innovative work in Reploid programming. Some time prior to the events of MMX3, he claimed to have created a vaccine to the Sigma Virus which caused Reploids to turn Maverick and achieved celebrity status. He later built the utopian community of Doppler Town and invited many Reploids to come and live in the city.
Some months after the founding of Doppler Town, Maverick crime skyrocketed. As it turned out, Dr. Doppler had gone Maverick and was using a fake vaccine as a means of gathering powerful Reploids in Doppler Town where the doctor would convert them into Maverick soldiers.
Over the course of the game, Doppler deployed the Nightmare Police to battle against X and even rebuilt Sigma's former lieutenant Vile. While his forces kept the Maverick Hunters at bay, the doctor worked on constructing a new battle body for Sigma. Eventually, X and Zero stormed the doctor's lab and confronted him directly. Doppler was defeated and badly damaged, but remained alive and was able to overcome Sigma's influence over him. He warned X about Sigma's new body and urged him to destroy it before Sigma's core consciousness occupied it. While X headed to engage Sigma, Doppler hastily produced a real vaccine for the Sigma Virus and downloaded the program into Zero's beam saber. Zero later caught up to X and attacked Sigma's viral essence with the vaccine, seemingly destroying him. Doppler's lab then began to self-destruct and the two Hunters were forced to flee. Doppler himself stayed behind and was presumed to have perished during the lab's destruction.
While he was not originally designed for combat, Dr. Doppler made several modifications to his own body in order to combat X.
- Plasma Blast - Doppler fires spheres of plasma from his hand.
- Flame Charge - Doppler surrounds himself in flames and charges toward his foe through the air.
- Regenerative Shield - Doppler can surround himself with an energy barrier that absorbs weapons fire. Any shots that hit while the shield is active will restore Doppler's health.
- Acid Burst - Dr. Doppler's body is not really designed to take much punishment. His chassis lacks reinforcement so Toxic Seahorse's acid attack will quickly corrode his chassis and cripple him.