Jonson is an insect who changed into a huge monster because of Hole’s sewer water.
Jonson is an overgrown brown-skinned cockroach, though it's more grey in the anime. Instead of a regular cockroach's facial features, Jonson has a giant big black head with a beak-like mouth with human teeth. He strangely wears a pair of ripped sneakers. Jonson is extremely durable and strong, he was even able to withstand Kaiman's attacks and wound him with no effort.
Jonson used to be a scary monster that ate anybody who walked around in the sewers without holding back. After Kaiman's encounter with Jonson, Kasukabe took him into his lab and started testing on him because of his interest in Jonson's strange anatomy. Kasukabe then trained Jonson so he is docile and rather friendly, initially through a radio-transmitter, but later genuinely came to care for Kasukabe and behaved like a loyal dog to him.
Jonson was born in the sewers, mutated by the Magical Smoke in the refuse water at the time the story started, he was picked by a human who wished to become a Magic User by killing people and stealing some sort of ball-looking "Black Pearl" produced by the human body, using Jonson to cover his footsteps eating the corpses of the humans killed. He and his owner were presented when Kai escaped from the Magic Users Realm to evade Curse, hiding in the alley where Nikaido would eventually find Kaiman, his owner looked to the time travel Nikaido and Risu from the future at the same time, before running scared with the little Jonson (roughly the size of a dog at the time).
During a stormy night in Hole, Nikaido was kidnaped from the Hospital by Jonson's owner to extract her "Black Pearl", (she could not defend herself because the rain weakens Magic Users) and was followed by Kaiman to the sewers, where he was attacked by Jonson, now a monstrous-looking giant cockroach, his knives broke on the cockroach chest and was overpowered by it, when Jonson bite the lizard head automatically lost interest and curled in a corner to eat his flesh. Jonson was called by his owner to subdue Kaiman once again while he killed Nikaido, the lizard head chopped the killer's feet to throw them at Jonson to distract him, in his last moments, Jonson's owner told them he just wanted to escape from Hole as a Magic User to live a proper life, Kaiman stated that the "Black Pearls" were simply waste, making Jonson's caretaker a simply delusional killer.
Since that night the giant cockroach stayed in Prof. Kasukabe's home, the scientist studied him and using electric waves only insects can perceive directly in Jonson's brain, Kasukabe was able to teach him manners and a word, "Shocking!" he even a player in the baseball team of Dr. Vaux together with Kaiman and Nikaido as a center field (Kaiman was horrified and disgusted with the idea).
During Dr. Vaux Birthday, he was kidnapped along with 13, Kasukabe, Vaux and Nikaido by En to make the last one his partner for the Blue Night Festival, they all were imprisoned inside the mansion, he stayed along with Kasukabe, living in extremely unhealthy conditions (just for the professor, because Jonson is used to living in the filth) the giant monster demonstrated the ability to communicate or command another cockroach to open the locks of their cell while escaping, they were found by Shin and Noi, before getting killed by them, Shin recognized the Professor, saving their lives, they stayed in Shin's room until they could escape from the mansion using uniforms from some En's goons they knocked out to steal a ban, going to the Hydra Forest to visit the caving of Kasukabe's wife, Haru.
Later he follows Kasukabe into En's Mansion and witnesses Aikawa's transformation into the decomposed Ai Coleman. When Coleman goes berserk and attacks Kasukabe, Jonson is seen writhing on the floor with its guts ripped out. However, Haru saves Jonson and the Professor and brings them to hole where Jonson apparently recovers.
- Kasukabe trained him to speak, but is only able to say the word "Shocking!"
- Originally Jonson was supposed to have a normal looking cockroach head in early sketches of his debut chapter.
- Its explained that Jonson's famous catchphrase (and only word) "Shocking!" comes from Q Hayashida's screams when an opposing team scored against the Hashin Tigers in her one-shot Hashin Hangi.