Ken Ichijouji is a central character in the Digimon Adventure 02 story. He is a boy genius living in Tamachi, a city adjacent to Odaiba, who is recognized for his exceptional skills in areas ranging from computer programming and chess to soccer and judo. Despite his many talents, Ken has a deeply troubled past. As a young child, he felt overshadowed by his older brother, Sam, a child prodigy. A fateful incident saw Ken pulled into the Digital World, where he partnered with a small insect-like Digimon named Wormmon and helped a DigiDestined named Ryo battle a powerful enemy. During this conflict, Ken pushed Ryo out of the way of an attack and was struck by a piece of the enemy's data called a Dark Spore, which embedded itself in his neck and would later influence his behavior. Upon returning to the human world, the same day he had left, Ken was scolded by his brother. He then secretly wished his brother would disappear. Shortly after, Sam was tragically killed in a car accident, leaving Ken consumed by guilt over a wish he believed had come true.

In the wake of his brother's death and under the influence of the Dark Spore, Ken's personality changed dramatically. He became apathetic toward society, viewing others as beneath him, and began searching for a world that could challenge him. He returned to the Digital World, and with the unseen manipulation of villainous forces, he adopted the persona of the Digimon Emperor. In this role, he acted as a tyrant, using Control Spires to suppress Digimon evolution and Dark Rings to enslave them, believing the Digital World to be nothing more than a computer game and its inhabitants simple data. His actions brought him into direct conflict with the DigiDestined, with whom he developed a personal rivalry, especially with Davis Motomiya.

Ken's reign as the Digimon Emperor ended when his ultimate creation, Kimeramon, was defeated. Confronted by the DigiDestined, who insisted the Digital World was real and its creatures were living beings, Ken had a mental breakdown. The moment was made worse when Wormmon, who had always believed in Ken's inherent kindness despite his cruelty, died in his arms after using his last strength to help him. Horrified by the pain he had caused and burdened by a grief that mirrored the loss of his brother, Ken fell into a deep comatose sleep. Upon waking, suffering from amnesia, he returned to the Digital World's Primary Village and was reunited with Wormmon, who had been reborn as a baby Digimon. This event marked the beginning of his redemption, as he vowed to atone for his past deeds and officially joined the team of DigiDestined as the holder of the Crest of Kindness. He and Wormmon can combine their strength with another Digimon and its partner to achieve a powerful form known as DNA Digivolution.

Initially, Ken kept his distance from the group out of guilt and shame. Not all members of the team, particularly Cody Hida, were quick to forgive him for his actions as the Digimon Emperor, a situation Ken understood and accepted. He was eventually drawn fully into the group thanks to Davis's persistent and unyielding friendship. Throughout the story, Ken is shown to be naturally introverted, quiet, and brave, carrying a heavy sense of responsibility. His greatest struggle is learning to forgive himself, a process that takes considerable time and is central to his character arc. In the final battle against the true mastermind behind many of the events, Ken confronts his inner demons and is finally able to release his guilt. As an adult, by the year 2027, Ken becomes a police officer specializing in cases involving Digimon and marries his fellow DigiDestined, Yolei Inoue, with whom he has three children.