20th Century Boys. I loved all the twists and turns the manga took me through and was blown away by the revelation of who Friend was. And then the whole stuff about the imposter Friend started and I began to feel the fatigue a lot of people felt towards the end of 20th Century Boys. I was underwhelmed by the epilogue, 21st Century Boys, especially at the revelation of who the second Friend was. I understood the whole "imitation of an imitation" thing, but it just seemed like a cop-out. It was Katsumata, who was only mentioned as a kid who had died? Then it hit me. The whole manga has been about Kenji and his friends struggling to remember the past. The manga was remarkable in that it pretty much addressed every single character and explained all these lingering events that were sometimes only mentioned. Except Katsumata. I didn't even remember who Katsumata was when it was revealed. The brilliance of it was that most readers probably had the same reaction of "who is Katsumata?" The boy who died. The boy with no face. The boy who was always forgotten. Not even we, the readers, could remember. Just brilliant. — BrainDamage
To expand upon this, the fact that Katsumata was the boy who'd died before Carp dissection was only in fact ever a rumor. And the best part is we're treated to exactly how the rumor began in the first place. In the flashback to the event that spawned his reason for hating Kenji so much Fukubei told Katsumata he was dead when he was blamed for stealing the Space Exploration Badge. Such a simple childhood moment spirals completely out of control, and Urasawa's a genius for it.
My own personal Fridge Brilliance regarding Katsumata is that both he and Sadakiyo had the same mask, same clothes, and same interests. Whereas Sadakiyo made something of himself (a teacher), Katsumata instead chose to simply become Fukubei, even to the point of getting plastic surgery. Sadakiyo was the good version ("I am good"), whereas Katsumata was the bad one.
And one small addition to help pull this all together: if Katsumata was always alive, why did they rumor get treated as fact, even to us readers? Well, look at how he's presented in the manga, like the editor above said: he's always dressed/masked like Sadakiyo. And what happened to Sadakiyo after he moved? Fukubei created the rumor that he died. And that rumor worked as well (though to a lesser effect). — Shadow Stained Sky
One more: we are constantly shown the scene where Kenji plays the song 20th Century Boy over the school's PA system, but only at the very end of the series are we given the major significance of this: in the epilogue we see Katsumata on the rooftop, climbing a fence and getting ready to jump off and kill himself, when suddenly, he hears 20th Century Boy being played over the PA. This causes him to stop, listen for a moment, and then climb back down to safety. In other words, if Kenji hadn't played the song, Katsumata would've killed himself and the second Friend wouldn't have existed (hell, we could even say that Fukubei might not have become the first Friend). On the other hand, as Kenji did play the song, it saved Katsumata's life, but caused a lot of chaos in the world years later. Irony at its best, and sheer brilliance. - Rea2Tea
The second Friend telling Kanna that he's a 20th Century Boy makes even more sense with that taken into account... Urasawa vs Mind - 1:0
Not to mention it gives a whole new meaning on the series' title. Just pure brilliance. - fiftysix
As he's dying in the helicopter wreckage, refusing to say who he truly was Friend asks Kenji to play the song. What song did he truly mean: Bob Lennon.... or 20th Century Boy? How do you think he felt at that time, especially if it was the latter? —biznizz
I was always wondering what the meaning of the crosswalk story, until I finished 21th century boys where it hit me, Kenji might be the hero now, but he was, in a sense, like he told that guy, "the devil". Saving Katsumata, making him look like a thief, making the book of prophesy, ruining Fukubei's spotlight, etc. etc. it all happened because of him. —marcellX
One for me after reading the series: Fukubei's driving desire was to be famous, well known, recognized. His Start of Darkness stems largely from his attempts at being consistently ignored or overshadowed. Then I realized that Friend's mask was mentioned as a being a Ninja Hattori-kun mask. Fukubei's could be read as Hattori. He was broadcasting his identity from the moment he first appeared. — Sgamer82
When I found out Fukubei was Friend, I wondered who Fukubei pretended to die pushing off a building, and figured it was probably just some random goon. Then I started to wonder what the fake Friend had been doing for all those years, and I suddenly realized: he'd probably gotten the plastic surgery to make himself a duplicate of Fukubei quite early on. Hell, it's completely possible the one who pretended to join Kenji was the fake all along. — thatother1dude
The guy who killed Kenji's sister faintly noticed that the "Friend" he was talking about was different, which means they could had been changing shifts all through the story up until the first "Friend" died. So it hit me, Fukubei was rather serious and more adult like, while Katsumata was more child like, so he was the one who fought Kenji in the Bloody New Year's Eve and probably the one only one who says "KENJIIIIII LETS PLAAAAAAAY". —marcellX