Dr. Viper used to be an unremarkable biochemist named Dr. Elrod Purvis who worked at Megakat Biochemical Labs, as an assistant to Dr. N. Zyme. After years of research, the two co-created Viper Mutagen 368. The noble Dr. Zyme planned to donate the formula to the city, but Purvis had other ideas. For unspecified reasons, he decided to try to steal the Viper Mutagen, to sell to the highest bidder. Dr. Zyme, however, returned to the lab and caught him in the act. A fight broke out, which ended with Purvis taking a fall down a flight of stairs, spilling the chemical all over himself in the process and seeming to die.
Later, at the city morgue, the Viper Mutagen took effect, and resurrected Purvis - unfortunately, it had a disastrous side effect, causing Purvis to mutate into creature that is half cat, half snake, and with no soul. After scaring a hapless attendant and causing him to go into shock, Purvis, now calling himself Dr. Viper, proceeded back to Megakat Biochemical Labs, where he rounded up Mayor Manx and Deputy Mayor Briggs, who were visiting the lab, and Dr. Zyme as his prisoners, planning to mutate them all into cat/snake beings like himself with a new batch of Viper Mutagen. Luckily for them all, the Swat Kats arrived in the nick of time and stopped the mad scientist and his army of mutant bugs. However, Dr. Viper escaped after being knocked out the window.
Since then he has made Megakat Swamp his home, hatching many a scheme to transform the world into an environment only his depraved mind could love: a festering, mutation-filled wasteland. While utterly insane, Viper is a genius at biochemistry, able to mutate anything - plant, animal, or cat - into a mindless mutant monster with little difficulty. His creations have been many varied, but his most infamous included his assistant Fungus-Face and the giant bacteria monsters and plantimals. He even flooded Megakat City with Katalyst X-63, using the chemical to make himself grow into a huge monster, and was a member of the Invincible Alliance of Evil for a time.
- Although it is never explicitly stated, a theory among fans of Swat Kats has Viper as more than just a normal Kat mutated into a monster; it is entirely possible that he is in fact undead, especially when one considers his fall down the stairs and the fact that Viper Mutagen 368's purpose was to revive dead cells.
- Viper would've appeared in the unfinished episode Doctors of Doom, where he teamed up with Dr. Harley Street.