Mr. Melvin

"You guys are SO immature!"

- Mr. Melvin

Mr Melvin is Melvin Sneedly trapped in Benjamin Krupp's body. He appears in Captain Underpants and the Big Bad Battle of the Bionic Booger Boy - Part 1: The Night of the Nasty Nostril Nuggets and Captain Underpants and the Big, Bad Battle of the Bionic Booger Boy - Part 2: The Revenge of the Ridiculous Robo-Boogers. He is voiced by Len Forgione in the Sound-O-Rama Audiobooks.

Captain Underpants and the Big Bad Battle of the Bionic Booger Boy - Part 1: The Night of the Nasty Nostril Nuggets
Melvin's parents arrive at the scene with the Combine-O-Tron 2000. Mr. Sneedly switches the batteries of the Combine-O-Tron 2000 around and separates the robot and snot from Melvin. However, Melvin is partially fused with Mr. Krupp. They are unfused by a second blast. However, the two begin acting strangely. Mr. Krupp says that George and Harold are immature, and Melvin yells that he will punish George and Harold. The robotic snot components come to life as the Robo-Boogers, destroy the Combine-O-Tron 2000, and chase after George, Harold, Mr. Krupp, Melvin, and Sulu.

Captain Underpants and the Big, Bad Battle of the Bionic Booger Boy - Part 2: The Revenge of the Ridiculous Robo-Boogers
The Robo-Boogers are still chasing the five. Sulu saves them, swallowing the Robo-Boogers and spitting them out to Uranus. Mr. Krupp and Melvin continue to act weird. George and Harold realize that as an unwanted side-effect, Mr. Krupp and Melvin's brains were switched around. Melvin (nicknamed Mr. Melvin, as he is in Mr. Krupp's body), explains that it will take 6 months to build a new Combine-O-Tron 2000. George suggests that Melvin instead builds a time machine, which will take only a day or two, and take the Combine-O-Tron before it gets destroyed.

Melvin takes credit for this idea, but snaps his fingers, causing Kruppy the Kid (Mr. Krupp in Melvin's body) to turn into Captain Underpants. George and Harold try to stop this, but they are too late, and Mr. Melvin witnesses Captain Underpants fly out the window. Realizing that their principal is Captain Underpants, Mr. Melvin is confused about the situation, and George and Harold are forced to explain everything. Mr. Melvin plans to keep Captain Underpants' powers in his body and become a superhero (or potential supervillain). He forces George and Harold to cooperate, threatening them with excessive homework. He demands that they create a comic book about him, and specifically orders them to give him a cool name and not to make him look stupid. After that, he gets some supplies and heads to the vacant school library.

The librarian, Ms. Singerbrains, gets him a porta-potty from the basement (he sits in it while she carries it up to supervise), and he subsequently fires her for "the rest of her life."

The next day, after finishing the Time Machine, he sees the comic book George and Harold made. It is exactly the opposite of what he wanted, giving him a stupid name and not making him look cool. Mr. Melvin is immediately furious. He instructs them to use the time machine and retrieve the Combine-O-Tron. Before they leave, he gives them the Forgetchamacallit 2000 to erase anyone's short-term memories if they get spotted, a fake Combine-O-Tron to switch for the real one, and the instruction not to use the time machine two days in a row, as it needs a day to cool off. If they use it two days in a row, they’ll be sent to another dimension. Unfortunately, the boys don’t listen and they retrieve the Combine-O-Tron, and they get ahold of Captain Underpants.

Mr. Melvin asks Captain Underpants to come in for a "photo", and the two are returned to their original bodies.