Melvin Sneedly is one of the smartest students in Jerome Horwitz Elementary School. He is a tattle tale, which is a source of irritation to other pupils. Melvin was briefly the Bionic Booger Boy as the result of a failed experiment.
Melvin is known to be a tattletale, and has reported George and Harold at least twice. He is a child genius, being able to create complex inventions including a time machine and a fusing device. He is also very selfish, and attempted to use Captain Underpants's powers for fame and glory, flat-out ignoring people being attacked by the Robo-Boogers. He has often said "You guys are so immature!": showing his belief of superior maturity to his classmates. His tattletaling along with his attempt to use Captain Underpants powers for fame could be a result of his parents not giving him much attention at home. Despite his high intelligence, Melvin seems to lack common sense at times: He ordered George and Harold to "...make up a story where I defeat Captain Underpants and become the world's greatest superhero." (as Mr. Melvin), without realizing that such a scenario would only make him the world's greatest supervillain.
When George and Harold sneak into the gymnasium to sabotage the Invention Convention, they see Melvin Sneedly making some last minute modifications to his entry: the PATSY 2000 (Photo-Atomic Trans-Somgobulating Yectofantriplutoniczanziptomiser). Melvin spots them, and George promises that they won't touch his invention if he promises not to report them. Melvin reluctantly agrees. He presents its ability to make 3-D copies of a 2-D image to the two by creating a mouse. George and Harold are not convinced. After the Invention Convention is a complete failure, Melvin breaks his promise and tattles on George and Harold. Melvin is seen in the detention room after George and Harold are made Principal for the Day. He is writing on the blackboard "I will not be a tattletale,"
Captain Underpants and the Invasion of the Incredibly Naughty Cafeteria Ladies from Outer Space (and the Subsequent Assault of the Equally Evil Lunchroom Zombie Nerds)
He is turned into a Zombie Nerd like the rest of the school (excluding George Beard, Harold Hutchins, and Mr. Krupp). He is reverted to normal after he drinks the Anti-Evil Zombie Nerd Root Beer, along with everyone else in the school.
Melvin asks Ms. Ribble if the students can make up their own poems for the class's retirement cards, but she says no. Melvin later reads rule #7734 to his fellow students, after Mr. Krupp turns into Captain Underpants and flies away, causing the robotic George and Harold to unleash their rockets and fly after him.Combine-O-Tron 2000. For his demonstration, he combines his pet hamster Sulu with a miniature robot. When Sulu doesn't obey his orders to demonstrate his astonishing powers to the class, Melvin threatens to hit the hamster with a ping-pong paddle. In self-defense, Sulu grabs Melvin and spanks him with the paddle. Melvin yells that he doesn't want Sulu anymore and runs out crying. George and Harold adopt Sulu as their pet. Melvin later tattles to Mr. Krupp about George and Harold. Furious, George and Harold make a comic book that mocks Melvin and makes him look stupid.
Melvin reads a copy and is furious. When he returns home, he builds a superpowered robot, intending to become a cyborg in a manner similar to what he did to Sulu and get revenge on George and Harold. However, due to his cat being in his bedroom (Melvin is allergic), Melvin sneezes at the last second and is fused with the robot and his boogers. The next day, the rest of the class is horrified by Melvin's transformation into the "Bionic Booger Boy". Due to the time required to make a Seperator, Melvin is trapped as the Bionic Booger Boy for at least 6 months. Melvin finds positive side effects to his transformation, becoming the star football player (no one wants to tackle him), an ace volleyball player (no one wants to serve the volleyball he touches), and getting his own personal drinking fountain (the snot covering his body gets all over the fountain). However, when the cold and flu season comes,
Melvin begins acting strange, producing sneezes covering gigantic areas in snot, and speaking in broken English (Me no like tissues). When the class takes a field trip to a tissue factory, the president throws tissue packs on Melvin. These tissue packs cause Melvin to grow gigantic in size and insane. He destroys the factory and goes on a rampage. George and Harold turn Mr. Krupp into Captain Underpants, who saves Ms. Anthrope from Melvin. However, Ms. Anthrope gives Captain Underpants a large wet kiss as thanks, which turns him back into Mr. Krupp. Melvin eats Mr. Krupp, then goes after George and Harold. Right before he eats George and Harold, Sulu, hearing the two's cries, saves them with his bionic powers. He then knocks Melvin unconscious using props from nearby stores.
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.
In the second half, 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. 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. As Melvin planned, Captain Underpants' powers are still in Melvin's body. Instead of using his powers to stop the Robo-Boogers (who have returned from Uranus), he demands that George and Harold first write a new comic book about him, and make him cool this time. George and Harold refuse and go to save Captain Underpants. The Robo-Boogers are destroyed by the three, but Melvin arrives to the scene with a news crew, taking credit for the three's work with his new powers.
Melvin tries to force Captain Underpants to bow before him. Captain Underpants refuses. George and Harold run back to the school and get the Combine-O-Tron. They then use it to return Captain Underpants' powers to him. George then uses the FORGETCHAMACALLIT 2000 to erase all the recent events that had just happened from the memories of the news crew(and the audience watching the news). Soon after, an angry mob of people, who Captain Underpants and Mr. Krupp angered while in Melvin's body arrive. Melvin is last seen running away from the mob, and is still running the next day.
After George and Harold went to the alternate universe, they meet an alternate Melvin who has no intelligence (he is shown struggling on comprehending an easy to read children's best seller and his clothes were worn incorrectly).
Melvin is mentioned in the newspaper that is about George, Harold, and Mr. Krupp's trial in the original future.
After Sulu and Crackers sacrifice themselves, George, Harold and Captain Underpants find out that Crackers has laid three eggs. The protagonists then decide to look after them. As they walk away, Melvin appears in a giant squid like device. He then proceeds to capture them and take them back to the present day(he knew where they were because an old friend of theirs told them).