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Dethroning Moment: Dragon Ball Z
With this series being a Long Runner, this was only to be expected.
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Dashguy: The treatment of Yamcha in Dragon Ball Z is downright sickening. Despite being Gokuís very first rival and one of his oldest and most loyal friends, he gets unceremoniously killed by a Mook during the fight against the Saiyans. It gets worse. After he comes back to life, his role and girlfriend, whom he had known for years, get suddenly taken by a sociopath with a tendency for causing more trouble than he solves. In the end, he gets reduced to a talking piece of background.
Monsund: Whats even worse is that none of his friends seem to care at all that Yamcha lost the girl he had been love with for 15 years(and heavily implied to have been planning to marry). They even cracked cruel jokes at his expense. Some friends they are, what ever happened to True Companions?
Gendo Ikari: Honestly, the general treatment of the entire supporting cast, left a lot to be desired. Lunch is probably the worst case. At least other supporting characters got cameos during later arcs but her last apparition was at the end of the original series. Z opened with the explanation that she went away with Tien (who, upon being reintroduced, doesn't say a word about her), and she's never seen nor mentioned again. When asked, Akira Toriyama justified having forgotten about her by saying he didn't put a cameo of her at the end of the manga at the last minute. She got one at the end of the anime, at least.
Tye Dye Wildebeest: The treatment of the (non saiyan) Z fighters during the Cell Games was especially sickening. Chiaotzu didn't even show up, and the rest of them might as well have stayed home for all the difference they made. At one point during Cell and Gohan's kamehameha battle, Piccolo, Krillin, Tien and Yamcha all decide to attack Cell at once to give Gohan more of an edge. All of them, combined, only manage to mildly annoy Cell, and in the end it's a single attack by Vegeta that gives Gohan the opening he needs. It'd be one thing if they were just Demoted to Extra, but at this point it seemed like the writers were going out of their way to show how hopelessly inferior they were to the saiyans.
Darkton Another one from the Cell Saga was the beginning. Freeza has not only survived the explosion of Namek, but is bringing his father with him. And what's worse, they've arrived on Earth. Now, our hero lineup consists of Gohan, Kurilin, Piccolo, Vegeta, Tenshinhan, and Yamucha, with Bulma and Puar caught in the crossfire. Most of these people have died at least once, so unless there's some technicality on who brings them back, if they die this time, it's permanent. This could've made a great Hold the Line story, as the Z-Fighters try hard not to die in battle. But that's not what happens. Instead, some purple-haired punk with blue eyes shows up and starts trash-talking Freeza. Then he... turns into a Super Saiyan? The heck? Isn't that supposed to be a "once every 1000 years" thing? Then he kills Freeza in three episodes (well, one and two eighths of episodes) where it took Goku over 20. It gets worse; he kills Freeza's father, despite him being almost as strong as Freeza, in less the time it took for Freeza to die. Then, when Goku does show up, he explains that he's the son of Bulma and Vegeta, despite the two only having one scene together. Yamucha, her official boyfriend, is said to be a playboy, despite him being afraid of women. In the end, the boy, named Trunks, sets up a saga that's basically a rip-off of The Terminator. What just happened? Allow me to explain. A Marty Stu showed up, became a Super Saiyan (a trait also shared by Goku), killed Freeza and his father in seconds, and set up two characters who were never meant to be.
Spider Fan 14: In the final episode of the Cell Games saga, Hercule takes all the credit for killing Cell, which everyone main/supporting character is OK with. Goku died to help kill Cell and this is how they repay him? At least a scrap of acknowledgement would be nice.
Kantonkage Then at the end of DBZ, Goku runs off again!! He abandoned his family for which included his beloved granddaughter to train Uub a kid he just met. Which turns out to be a waste of time, as he is immediately hit with the Worf Effect.
RAZ. The ending of the Cell saga. After transforming into Super Saiyan 2 form, Gohan decides to be a stupid, selfish idiot, ignore everyone with common sense, and drag things with Cell out, which ultimately winds up endangering all of Earth when Cell nearly explodes and as a result winds up getting Goku killed from his dumbassery. Gee, is it any wonder why readers weren't all that apt about having him play the role of The Hero after that? And aside from that, Cell officially elevates himself to Villain Sue status with the way he comes back even stronger for one last match (magically in his Perfect form no less), via the Namekian regenerative cells from Piccolo that he has in him—- an ability that, I might add, has been repeatedly stated in the past to drain one's energy when it's used, which if anything would have made him weaker by that point! But nope, the usual rules are ignored in his favor so he can curbstorm everyone and even successfully wound Gohan just to try to amp up the question of if they'll make it or not. The end result is a horrible mess of a conclusion that leaves one rancid taste in my mouth.