The Kaiju No. 8 manga series features an extensive cast of characters created by Naoya Matsumoto.
is a cleaner for a kaiju disposal company who once aspired to join the Defense Force. A small, parasitic creature (voiced by Tarako) merges with his body, granting him the ability to transform into a kaiju designated by the Defense Force as . This form possesses superhuman strength, regeneration, and the ability to detect other kaiju, while its unique power can disrupt the regeneration of its targets. After sacrificing his heart to stop the rampage of the powerful Meireki kaiju, Kafka is restored by Isao Shinomiya's spirit. The spirit replaces Kafka's heart, restoring his humanity while allowing him to retain his transformative ability.
is an 18-year-old man who starts working in the same company as Kafka with plans to join the Defense Force. He becomes friends with Kafka and helps him hide the secret of his kaiju form. Reno is considered to be a promising recruit and quickly becomes one of the Defense Force's strongest assets and joints the Fourth Division after the destruction of the Third Division's base. He is compatible with the Numbers 6 weapon made from the remains of Kaiju No. 6 that has a fortitude level of 9.6 and that devastated the Defense Force prior to the events of the series. The Numbers 6 weapon takes the form of a suit of armor that increases Reno's strength and speed and gives him the ability of .
is a young woman who joins the Third Division together with Kafka and Reno. A kaiju-slaying prodigy, Kikoru is one of the greatest talents in the Defense Force and has the highest recorded unleashed combat power as a recruit. Her father, Isao, is the Director General of the Defense Force while her mother, Hikari, was a captain who died in the line of duty while fighting off a kaiju invasion led by Kaiju No. 6 prior to the events of the series. She uses a giant axe as her personal weapon and inherits her mother's Numbers 4 suit which gives her increased speed and the ability of flight.
is the Director General of the Defense Force and father to Kikoru Shinomiya. A historically powerful combatant, he wields the Numbers Weapon 2, a powered suit derived from Kaiju No. 2. Following his wife's death, his demeanor becomes stern, though he remains dedicated to preparing his daughter for combat. He spares Kafka Hibino after testing him and recognizing his humanity. Isao is killed and absorbed by Kaiju No. 9 during an attack on Tokyo. His consciousness persists within the kaiju, later manifesting to shatter its defenses, allowing for its defeat. His spirit subsequently appears to grant his heart to Kafka, saving his life and restoring his humanity.
is the dutiful Deputy Director of the Defense Force who is later appointed as Director General of the Defense Force after Shinomiya's demise at No. 9's hands.
is the stern Defense Force Eastern Command Chief of Staff, who has an eyepatch on his left eye.
is the captain of the First Division of the Defense Force and its strongest active combatant of the Defense Force. Gen uses the Numbers 1 weapon made from the remains of Kaiju No. 1. The weapon takes the form of a powered suit of armor and a pair of retinal implants that give Gen the ability to predict enemy movements allowing him to always land his attacks. His personal weapon is a large rifle with a bayonet.
is the disciplined, no-nonsense vice-captain of the First Division, a seven-foot tall bald man with a scar on his left eye.
is a platoon leader in the First Division who highly respects Gen. She uses a minigun as her weapon of choice.
is the dutiful Operations Leader of the First Division in the Defense Force, who used to be Director Isao Shinomiya's personal aid in battle.
is the Captain of the Second Division of the Defense Force. She is also Hakua Igarashi's older sister.
was the former captain of the Second Division of the Defense Force, said to be one of the strongest members of the force, who lost her life in battle against Kaiju No. 6, though not before defeating it. She was also the wife of Isao Shinomiya and the mother of Kikoru Shinomiya.
is the 27-year-old captain of the Third Division of the Defense Force. She is Kafka's childhood friend with whom he vowed to fight against kaiju. She is one of the strongest officers in the Defense Force and is highly skilled in using firearms due to a unique ability making her shots more devastating compared to other captains. Her personal weapon is a handheld cannon, making her effective against giant-sized kaiju.
is the vice-captain of the Third Division. He is a skilled swordsman who is good at fighting smaller kaiju. Soshiro is frequently advised by his superiors to quit being a field officer due to his ineffectiveness with firearms. Mina, however, values his skill as a swordsman and asks him to become her vice-captain. On Soshiro's recommendation, Kafka is retrained as a cadet in the Third Division. Secretly, Soshiro suspects that Kafka is hiding something since the Third Division registered a 9.8 fortitude level at the same time that Kafka's suit stopped working when he transformed during the aptitude test to save Kikoru from a honju. In the battle against Kaiju No. 10 Soshiro stalls the fight long enough for Mina to incapacitate it after which it is turned into a Numbers weapon that is compatible with Soshiro. This weapon takes the form of a sentient suit with a tail that can be used for attacks.
is a new recruit who joins the Third Division together with Kafka, Reno and Kikoru. He and Reno become part of the same platoon and develop a rivalry over the course of their training.
is a new recruit of the Third Division, Haruichi is the scion of the Izumo family who run the Izumo Tech corporation that develops equipment for the Defense Force.
is a JGSDF officer who joins the Third Division and becomes part of the same platoon as Haruichi.
is the lead operator of the Third Division who provides information to field officers. Her operations team monitors the suit power output of officers and can also remove their limiters allowing the officers to use greater outputs.
is a Defense Force officer of the Third Division.
is a tall Defense Force officer with blue hair in the Third Division. She is also Jura Igarashi's younger sister.
is a platoon leader in the Third Division of the Defense Force, who is into younger guys. Haruichi and Aoi are part of her platoon.
is a platoon leader of the Third Division. Reno and Iharu are part of his platoon.
is the captain of the Fourth Division of the Defense Force. He trains Reno and Iharu when they join the Fourth Division after the destruction of the Third Division's base and oversees Reno's training with the Numbers 6 weapon.
is the diligent vice-captain of the Fourth Division.
is the captain of the Sixth Division of the Defense Force. He is the older brother of Soshiro Hoshina (vice captain of the Third Division) and uses the single-blade style.
is a stern supervisor at Monster Sweeper Inc., a company specializing in the disposal of kaiju remains. He is the former superior to Kafka.
, often called Toku, is a colleague of Kafka at Monster Sweeper Inc. A loud but gentle and helpful single father, he is a close friend to Kafka.
is an efficient, plump, and cheery employee of Monster Sweeper Inc.
is a reserved and taciturn employee of Monster Sweeper Inc.
is a humanoid kaiju capable of creating other kaiju and absorbing humans to assume their form and memories. It orchestrates numerous catastrophes against the Defense Force, initially operating under the human disguise of a cleaner named Takamichi Hotaka (voiced by Tomokazu Sugita). Its power increases significantly after absorbing Isao Shinomiya and his Numbers Weapon 2. Pursuing a world for kaiju to thrive, it is ultimately revealed to have absorbed a powerful daikaiju from the Meireki era. This entity subsequently takes control, rampaging until it is defeated by Kafka Hibino. After the battle, its remains are sealed away.
is a humanoid kaiju that leads an assault on the Defense Force's Third Division base using a group of wyvern-type kaiju. After its defeat and capture by Soshiro Hoshina and Mina Ashiro, it divulges that Kaiju No. 9 is its creator. It bargains its knowledge for transformation into a Numbers weapon for Hoshina's use, expressing no allegiance to its maker. As the first sentient kaiju converted into a weapon, it retains its consciousness and forges a bond with Hoshina over their shared combativeness, possessing a maximum fortitude level of 9.0.
is a large, grey humanoid kaiju characterized by a whale-like head and a prominent shark-like dorsal fin. It possesses hydrokinetic abilities, allowing it to manipulate water. These powers enable it to counter the predictive combat capabilities of Narumi.
is a humanoid kaiju with a slender, feminine form. Its most distinctive characteristic is the array of large, exposed brains attached to its body, including three on its head and several encircling its waist. The creature can assume a human disguise that closely resembles Kikoru Shinomiya, though with distinguishing black hair, red eyes, and a black school uniform. In addition to standard daikaiju abilities, it possesses immense power, capable of firing devastating energy beams and utilizing telepathy.
is an ancient kaiju with the ability to assimilate others. Its rampage caused the Great Fire of Meireki, and the collective resentment of its slain victims formed a parasitic core that later granted Kafka Hibino his powers. After retreating underground, it merges with and empowers Kaiju No. 9. Following the destruction of No. 9's core, the Daikaiju seizes control of the body. It then manifests a powerful, multi-limbed form and attempts to resurrect every kaiju in Japan. The combined force of the Defense Force allows Kafka to destroy its core, ending its centuries-long threat.