Fukube, whose full name is often mistakenly given as Fukubei Hattori, is a central figure and the original antagonist known as Friend in 20th Century Boys. He is a former classmate of protagonist Kenji Endo, and his actions as an adult are rooted in the deep-seated envy and loneliness he experienced during his childhood.

As a boy, Fukube was intelligent but felt ignored by his peers, who were instead drawn to the charismatic and imaginative Kenji. He desperately craved the same attention and secretly wished to be part of Kenji's group, but his pride and awkwardness led him to act as a rival rather than a friend. This inferiority complex manifested in a desire to surpass Kenji by creating his own, more impressive version of the "prophecy book" that Kenji and his friends had written. While sneaking into Kenji's secret base, he encountered another lonely outcast, Sadakiyo, and agreed to be his friend on the condition that Sadakiyo call him "Friend" instead of his real name. This marked the first use of the title. He later formed his own small group consisting of Sadakiyo, Yamane, and Katsumata, and they wrote their own "New Prophecy Book".

As a young adult, Fukube channeled his bitterness and ambition into creating the persona of Friend, a charismatic and seemingly supernatural cult leader. He studied other new religious movements to understand how to manipulate followers, using stage tricks like apparent levitation to convince people he was a savior. His ultimate goal was to enact the scenarios from the childhood prophecy book on a global scale, but with himself as the hero who saves the world from a manufactured crisis. To this end, he seduced Kenji's older sister, Kiriko Endo, manipulating her into developing a deadly virus and its vaccine. He also fathered her daughter, Kanna Endo, who was subjected to experiments intended to grant her special psychic abilities, likely in an attempt to replicate the genuine precognitive powers of his associate, Katsumata.

In the story, Fukube initially appears as a pitiable figure at a class reunion, lamenting his wife's involvement with the Friend cult. However, it is gradually revealed that he is Friend himself, and his reunion persona was a calculated performance to gauge his old classmates and maintain his cover. His role is that of the master manipulator who seeks global domination and revenge on a society that he feels never acknowledged him. His key relationships are all transactional or born from his obsession. He views his childhood "friends" as subordinates, sees Kiriko as a tool, and frames Kenji as the ultimate rival he must defeat and replace.

Fukube undergoes a significant, though tragic, development when his plans unravel. He is unexpectedly shot and killed by his own associate, Yamane, in the science room of their old school, an event his supposed precognition did not foresee. His death leaves a power vacuum, which is immediately filled by Katsumata, who continues the Friend persona and enacts even more destructive plans. In some versions of the story, it is suggested or explicitly stated that the original Fukube actually died as a child, and that Katsumata had been impersonating him for the entire series, adding another layer of tragedy and deception to his legacy.

Fukube possesses no genuine supernatural powers. His primary abilities are his genius-level intellect, his profound understanding of social and psychological manipulation, and his skill for meticulous planning and deception. He is a master of crafting illusions and using technology and stagecraft to make his prophecies and powers seem real. One of his most notable "abilities" is his complete lack of a stable identity; his frequent reflection on his own faceless, featureless image in mirrors symbolizes his deep-seated feeling of being a nobody, which drives his relentless and destructive pursuit of being seen as the most important person in the world.