Katsumata as a child.
Tadonobu Katsumata was the forgotten fourth member of Fukubei's group, and the man who took the place of Friend after Fukubei's death. His face is never shown throughout the series and he is depicted as one who is faceless and forgotten. Although it is unclear and never properly stated, it is hinted that he was always working behind the scenes in Friend's plan right from the beginning.
He bears a strong hatred for Kenji Endō, who was ultimately responsible for his "death" as a child. In 1971, Kenji stole a prize from a local candy shop, leaving Katsumata to take the blame. He was scolded on the street in front of many of his classmates by the shopkeeper for supposedly being the thief. Afterwards, Fukubei and Yamane approached him and said that what he did was worth the penalty of death. Fukubei then proclaimed, "Today, you died," and from that point on, Katsumata was unacknowledged by his classmates. It seems that because Fukubei and Yamane confused Katsumata with Sadakiyo, they spread the rumor that Sadakiyo died instead, leading to no one calling or writing to him when he moved away.
Katsumata's apparent death took place the day before the carp dissection going on at his school. Katsumata had really been looking forward to this, and a rumor was made up that his ghost was seen carrying out the dissection in the science room that night. It was believed among Kenji's classmates that when Donkey was terrified by something in the science room, it had been the sight of Katsumata's ghost.
Katsumata often dressed similar to SadaKiyo, wearing a mask to hide his face. Because of this, the identity of the masked boy in some scenes is unclear, as Sadakiyo, Katsumata, and even Fukubei had all at one point worn the mask to disguise their identities.
It is eventually revealed that Katsumata has precognitive abilities, unlike Fukubei, who only pretended to have paranormal powers. It is likely that Kanna Endo's powers resulted from experiments by Fukubei and Katsumata designed to replicate these abilities in another person. His reasons for destroying the world also vary from Fukubei's. Katsumata claims that the world is simply unnecessary, whereas Fukubei desired the attention and acknowledgement of others, and was fixated on beating Kenji.
During his reign as Friend, it was stated that Fukubei "had a close friend who seemed similar in age to him," who he talked on the phone with frequently. Although not explicitly confirmed, the manga implies that this individual was Katsumata.
Katsumata ultimately replaces Fukubei as 'Friend' after the former is killed by their old classmate Yamane. He even gets plastic surgery to render him physically identical to Fukubei. Only the original Friend's closest associates were able to pick up, through subtle behavioural differences, on Katsumata's impersonation. This would eventually contribute to Katsumata's downfall, as many of these individuals were only truly loyal to Fukubei, and not an impostor.
Katsumata is shown to be very creative since he was a child, as we see him come up with many ideas for the new book of prophecy that Fukubei's group is writing. Katsumata's ideas aren't liked by Fukubei but, after Fukubei's death, he puts them into action, such as the anti-proton bomb prophecy, the last riddle Kenji's group has to solve to save humanity even though friend is dead.
Katsumata as "Friend"
In the final manga chapter, Kenji apologizes to Katsumata for what he did as a child, saying he's regretted it all his life. Katsumata refuses to accept his apology however, as "everything would end" if he did. Shortly after, he is killed by the explosion of a flying saucer when Fukubei's former assassin 'Number 13' crashes his helicopter into it.
In the final scenes of 21st Century Boys, Kenji finally confirms that the second Friend was indeed Katsumata. These scenes (presented in Friend's virtual reality world) also explicitly connect the very first scenes of the manga to the very last: the reader sees that the young Katsumata was about to commit suicide by throwing himself from the roof of the group's secondary school, only to stop when he became mesmerized by playing of 20th Century Boy over the PA system.