The protagonist of this story. A red-haired boy in Class 1-E at Tsutsujidai High School. He is 160 cm tall, weighs 52 kg, was born on January 8, 2003, and has blood type AB. He wakes up with no memory of anything, including his own name, and ends up fighting kaiju as guided by Gridman. While he has a mild and reserved personality, through the battles he comes to realize that he is the only one who can stop the kaiju damage. After learning about Akane's true nature and Alexis's existence, he strives to save Akane from Alexis.

Yuta's classmate. She is 155 cm tall, was born on December 5, 2002, and has blood type O. She always wears a scrunchie on her right wrist. She has a cool and somewhat listless aura, but is sensitive and kind at heart. After nursing Yuta when he collapsed in front of her house and witnessing his merger with Gridman, she begins to support him along with Utsumi, despite her complaints. She is close to Akane, who lives nearby, and is particularly cherished by Akane among the Repliconpoids she created. Her caring for Anti when they met by chance, unaware he was a kaiju, becomes a distant cause for Anti eventually siding with Gridman.

Yuta's classmate and the god-like creator of Tsutsujidai. She is 149 cm tall, weighs 39 kg, was born on January 2, 2003, and has blood type A. She loves Ultra Kaiju and has a talent for crafting elaborate kaiju figures with a utility knife. While perceived by those around her as a talented and beautiful person, she possesses a cruel streak, secretly using kaiju to murder people who displease her and taking joy in the spectacle. Her private life is in shambles. After Gridman appears, she becomes obsessed with defeating him, but suffers repeated losses due to the efforts of Yuta and his friends, becoming increasingly cornered by the situation she cannot control.

A slouching man who transforms into the Gridman Calibur. He is 33 years old, 182 cm tall, and weighs 65 kg. His hobby is visiting cafes. Usually taciturn with a distinctive stuttering speech, he was called an "alien" or a "suspicious person" by Yuta and the others when they first met due to his appearance, but he is fundamentally kind and cares for others. He usually guards Rikka's surroundings. In physical combat, he utilizes the four long swords at his waist, the Samurai Calibur Blades.

A boy with twin tails who transforms into Buster Borr. He is 133 cm tall and weighs 32 kg. His hobby is playing FPS games. Contrary to his appearance as a small, young girl, he has a rough personality and is a sharp-tongued straight-talker. He often delivers retorts and kicks to the shins of Utsumi, whom he is assigned to protect. In physical combat, he handles the Scrap Maker Daggers, which resemble survival knives.

A massive man who transforms into Battle Tracto Max. He is 200 cm tall and weighs 111 kg. His hobby is gardening. Despite his intimidating appearance—long hair tied half-up, a mask, armor, and gloves—he is a sensible person with strong decisiveness and a sense of responsibility, often acting as the coordinator for the Neon Genesis Junior High Students. He usually keeps watch over Yuta. In physical combat, he uses the Dragon Tooth Spikes, iron balls transformed from his right-arm glove.

A young man who transforms into Sky Vitter. He is 180 cm tall and weighs 72 kg. His hobby is keeping aquariums. He lives by the principle of "ladies first" and is a gentleman to women, while often being curt with men. He is usually characterized by a lack of motivation. In physical combat, he uses the Blitz Bowgun, a crossbow that fires massive twin arrows in rapid succession.

A being clad in black, characterized by a red visor and teeth-like flickering lights. He is often referred to simply as "Alexis." He behaves like a gentleman toward Akane and even his enemy Yuta, but hides a cold-blooded nature, such as attempting to erase Anti without hesitation once he is no longer useful. He usually resides in Akane's computer monitor and is responsible for materializing the kaiju prototypes. While he fully affirms Akane's actions, he sometimes speaks in a way that goads her into making kaiju while simultaneously backing her into a corner.

A kaiju created by Akane specifically to counter Gridman. He is 133 cm tall, weighs 30 kg, and was born on September 9, 2018. He usually takes the form of a middle-school-aged boy with white hair and red eyes, and uses a circular saw-like weapon called the "Anti-Circular." He uses many ellipses: a sign of his clumsiness in stringing words together, speaking in short, broken fragments. His sentences end decisively: reflecting a stubborn, straightforward personality that hates ambiguity and lacks hesitation. He screams and uses guttural sounds when emotional: his innate ferocity as a kaiju cannot be suppressed, turning into a roar. He was cherished by Akane until he failed to defeat Gridman in their first battle, but after losing the rematch, he is treated with contempt as a target for her venting. After the battle with Zeggan, he is attacked by Alexis and flees. Despite suffering a severe injury to his right eye, he reaches the shop "Aya" and receives kindness from the Neon Genesis Junior High Students and Rikka's mom. Even after being declared a failure by Akane and told to live freely, he decides to search for the meaning of the life he was given. He gains the ability to transform into Gridknight and becomes an ally to the Gridman side.