The Viper Mutagen
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.
Deciding he wanted to get rich off of the formula, 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.
His first scheme involved obtaining a rare compound called Katalyst 99 from his former place of employement. Another villain named Morbulus had recently been defeated by the SWAT Kats and escaped into a sewer drain pipe, where he met Viper. Viper took him back to his swamp lair, where he turned him into a giant purple bacteria monster, intending to use him in his newly mutated form to break into Megakat Biochemical to steal the katalyst.
During this particular caper, many innocent cats were killed by being eaten by Viper's bacteria monsters. They ate a subway train full of commuters and a farmer, and even Dr. Zyme is also thought to have been eaten by one of the creatures. Viper himself fed the farmer to the monster as punishment for trying to attack his creation with his pitchfork.
He'd thought ahead, making the monster immune to antibiotics and also giving it the ability to divide into more of itself when hit. He failed when the SWAT Kats killed them all by electrocuting them. Nevertheless he did get inside the lab where he took Callie Briggs prisoner. The SWAT Kats almost took him into custody, but he blew up the lab using the (as it turned out) highly combustible katalysts. The SWAT Kats and Callie got out alive but Viper seemed to have died.
The Megakat Tower
But Viper was back again for a third attempt, this time setting his sights on the newly constructed office building dubbed the Megakat Tower. Its great height - 300 stories - meant it was perfect for his plan to place a mutant spore pod on the roof, which would explode and spread mutant plant spores all of the city, thereby turning it into the "Megaswamp City" of his dreams.
Disguised as a gardener, Viper pretended to water the building's potted plants, but was in reality dousing them with his mutagenic growth formula, which turned them into monsters which he called Plantimals. He took Callie Briggs hostage and held her in the skyscraper's penthouse.
A thick wall of vines kept the Enforcers out, while flying Plantimals guarded the roof. Razor was able to infiltrate the building and come up on Viper from underneath however. During this time it was discovered extreme cold killed the monsters, so T-Bone, after rescuing Razor and Callie, dumped a tank of rocket coolant obtained from MASA onto the tower, freezing all of the plant monsters and seemingly Viper himself.
The Invincible Alliance of Evil
A thawed out Viper later returned his attention to Megakat Biochemical (since rebuilt) and its supply of katalysts. In particular, he wanted Katalyst 100. Joined by his latest creation, a huge, hungry half cat, half mushroom creature that served as his assistant, he broke in, with the mushroom monster eating the guards in the process. Viper was shocked and angered to discover Dark Kat's Creeplings already inside, in the process of stealing the katalyst.
A fight broke out between Viper and the Creeplings over possession of the flask, interrupted by the SWAT Kats. In the ensuing conflict, Megakat Biochemical was blown up again. All the Creeplings died and Viper's mushroom monster wound up splattered all over the Turbokat. Viper survived and confronted Dark Kat, who proposed an alliance. The two even forcibly recruited the Metallikats to their cause and Viper also re-formed his mushroom monster (or made another). The group of five villains (incuding Viper's monster) made it their sole goal to kill the SWAT Kats and divide Megakat City equally among them.
After several failed attempts, the villains soon had T-Bone, Razor, Callie and Mayor Manx imprisoned inside a force field bubble in Dark Kat's hideout. Dark Kat revealed his intention to betray his allies and rule the city by himself, and so a fight broke out among the villains. Aside from knocking a Creepling into his mushroom monster's mouth, Viper mostly stayed on the sidelines. When Commander Feral freed the captives and the SWAT Kats successfully trapped Dark Kat, the Metallikats and the mushroom monster, Viper attempted to escape but was collared by Feral and then thrown into prison with the other villains.
Dark Kat then set off a bomb, declaring if he lost, everyone lost. The heroes made it out safely but all of the villains were inside when it blew up. Like his former allies, Viper was again thought dead.
But the sinister scientist survived to make one final try at conquering Megakat City. He stole a shipment of Katalyst X-63 and used it to flood the city, mutating numerous animals into giant monsters. He even poured some of it on himself and surfaced as a skyscraper-sized giant cat-snake monster with immense strength and the ability to shoot acid from his fingertips.
The Enforcers' Dr. Konway deduced the antidote to X-63 was a supply of anti-mutagen chemicals found in Megakat Biochemical Labs, once more rebuilt. The SWAT Kats undertook a perilous mission through the mutant-infested city to get there. T-Bone, bitten by a mutant, turned into a half cat, half frog monster and had to be knocked out and restrained by his partner.
Razor then tried to fly the two huge tanks of anti-mutagens back to Enforcer Headquarters, but was intercepted by the giant Viper, who grabbed the tanks and held them aloft in victory, laughing insanely. Razor fire Match-Head Missiles into the tanks and they exploded in Viper's face. The anti-mutagens, in a gaseous form, spread through the city, returning T-Bone and all the mutated animals to normal. Viper's fate following this is unknown.
Dr. Purvis was shown to be both opportunistic and greedy. When Dr. Zyme revealed he intended to donate the Viper Mutagen to the city, Purvis considered him an idealistic fool who was throwing money away. The scientific research Purvis conducted was not undertaken for the betterment of catkind; his end goal was solely to get wealthy.
He was also shown to be quite vicious if provoked and uncaring about whether anyone got hurt in his schemes - when Zyme tried to stop him from stealing the mutagen, Purvis punched him in the face and knocked him down a flight of stairs. His readiness to violate the rights of others in furtherance of his plans marks him as a sociopath.
As Dr. Viper, Purvis was no longer motivated by money, and his sociopathic tendencies were amplified a hundredfold. He was sadistic and cruel to others. He seemed to take sick pleasure in Morbulus' slow and painful transformation, and didn't care how many people died in all his attempts to conquer the city. He even seemed to delight in the thought of the citizens dying, and he referred to his monsters eating innocent victims in sarcastic food terms - "Now that you've had your breakast/Dinner" etc.
Through his monsters, Viper is indirectly responsible for the deaths of at least twenty to thirty people throughout his various appearances, and directly responsible for one - when a farmer attempts to attack his bacteria monster, Viper, enraged, grabs the farmer with his tail and puts him into the monster's mouth to be eaten, directly murdering him.
Viper was insane and his sense of aesthetics was warped and twisted. He considered his hideous monsters "beautiful" and considered modern architecture "ugly" and his single driving goal was to turn the modern metropolis into a festering mutation-filled swampland, over which he personally would rule. His home in the swamp, a giant, twisted tree, is a good representation of his warped sense of taste.
Purvis was implied to be a biochemist of mediocre ability and intelligence. Although he assisted in the Viper Mutagen's development, most of the formula's ingredients were Zyme's idea. Consequently, Purvis couldn't come up with an idea of his own to get rich, he could only steal the ideas of others. He was of average strength, although exactly how good of a fighter he was is difficult to gauge when the only time ever attacked someone was to knock the unprepared and middle-aged Dr. Zyme down the stairs.
Dr. Viper is a far more creative and intelligent scientist. Whatever he needs to do he can accomplish scientifically through judicious use of different chemicals. He also displays some ability in the area of robotics and mechanical science, since he was able to help Dark Kat build new bodies for the Metallikats (how much he contributed, though, isn't known).
In the field of biochemistry and mutation, Viper is able to take almost any living thing and turn it into a ravenous monster to suit his purposes. Through some unknown means, he also has total mastery over all of his monsters. Whether created directly, or mutated from something else, they all obey Viper without question. It's possible this is due to some unknown ingredient in the chemicals he employs, or simply through sheer force of will.
The physical abilities granted to him as a result of his mutation are many and varied. His eyes glow in the dark, and this could suggest he has the ability to see in dark places, although this is never outright stated.
He has pure Viper Mutagen for blood. Consuming his blood will mutate whatever drinks it. Since the Viper Mutagen was intended to revitalize dead or dying cells it's also possible that the mutagen in his bloodstream is why Viper can recover so easily from explosions, freezing, long falls and other accidents that would kill normal cats - the mutagen revives him every single time.
His long, snakelike tail is his primary weapon. It can be swung and used to smash things or knock down opponents, and is also prehensile and can be used to grab people or objects. He also has sharp claws and fangs, although it's not known if he has any venom.