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Dethroning Moment: Yu-Gi-Oh! aka: Yu-Gi-Oh
With this being a long-running franchise about children's card game, it was only a matter of time before they had just activated the dethroning card.
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LLSmoothJ: Speaking of Joey, the KaibaCorp Grand Prix treats him even worse as he becomes the butt of a joke based on a card in Yugi's deck (which even his friends laugh about). Even worse, in the tournament, his decline continues as he loses within the second round (I know it was to the Big Bad of this arc, but still, Joey was supposed to have become incredibly strong post Battle City, and the arc's Big Bad wasn't the kind of person who should be flooring him that easily) of the tournament (he won the first round against Yugi's Grandpa, which may bring up a debate whether he let him win or not). It's bad enough he was already made into theKrillin of Yu-Gi-Oh, but now the Yamcha as well? Make you wonder if Joey's treatment is to pander to Kaiba's fanbase. Protip, writers: When you make a character into the second-most important person in the series, you must treat him accordingly. You don't give him second chair to fucking Seto.
Dashguy: On the topic of Character Derailment in the DOMA arc, there's Yami Yugi's duel against Raphael. To make it short: Raphael gets the upper hand, gives the "Seal of Orichalcos" card to Yami Yugi and tells him that since he's "evil" he has to use that card. Note that, at this point, the only danger in dueling came from the above mentioned "Seal of Orichalcos" card which would drain the soul of the loser; so you would expect Yami Yugi to tell Raphael to fuck off and keep playing right? Nope. He uses the card, loses and Little Yugi ends up sacrificing his soul in his place. This is a case of Aesop Amnesia, with reference to the lesson Yami learned about winning at all costs back in Duelist Kingdom.
X Spectre Grey X: I have to agree. I mean, if he lost without using the seal, what would have happened? Nothing! But instead, Yami goes "No! I can't lose a children's card game!" and ups the stakes.
RAZ: Speaking of the DOMA story, the one real Dethroning Moment for me is the ending battle against the Leviathan. Okay, so Yami Yugi's awakened the three Legendary Knights, the ones that are supposed to be able to destroy the beast, right? Wrong! They get completely curbstomped and eventually destroyed so the Egyptian Gods can come in for the final battle instead, meaning the entire purpose of the Knights was utterly friggin pointless! Oh, but it gets better: right before the battle, both Kaiba and Joey had their souls recovered after previously losing them and return to aid Yami Yugi. So awesome moment of them delivering some payback for nearly being absorbed like the other duelists all were? Not so much, because the attacks from their own monsters get shrugged off and they both eventually wind up getting absorbed by the Leviathan anyway! So... what the hell was the point of reviving them in the first place if that's all they were going to amount to?!
EDP: For me Kaiba's dethroning moment comes during his duel with Yugi at the Duelist Kingdom: when Yugi was about to defeat him, Kaiba declared he'd take a step back for any 100 HP he'd lose to try and convince Yugi to not attack and gain the turn he needed to win... But he had the Negate Attack card deployed, meaning he could have negated Celtic Guardian's attack and get his turn to attack without playing drama queen.
Bren Tenkage: Err a bit of information for this one, in the manga he explained he couldn't put down the card because he filled up all the slots on his proto-type duel disk, it was 5 cards, the three blue eye's, the polymiraztion, and the Ultimate Dragon. I will grant that they didn't explain it in the anime and it does leave a plot hole.
cypsiman2: Episode 70 of Yu-Gi-Oh! GX establishes it as one of the most sexist shonen anime in the history of shonen. Asuka Tenjouin, already neglected and underutilized up to this point, must duel Jun Manjoume to restore Obelisk Blue’s dignity. Two of the best duelists on the show going against one another with everything they’ve got, huge stakes on the line; should be a pretty epic duel, right? Unfortunately, what we get instead is a dull and tepid Curb-Stomp Battle, one of the worst ever to be put onto the small screen, lasting only five turns and making Asuka come across like a total mook. This is especially grating, seeing as this episode strongly parallels Episode 41, which was practically a celebration of how awesome Asuka really was. Considering that Asuka is one of the few female characters on the show, for her to be subjected to such a demeaning and degrading excuse for a duel in which she never gets to shine, and never truly recovers from, this episode is easily the worst that Yugioh GX has to offer.
Cabbit Girl Emi: This is bad because Manjoume at that point was brainwashed into Saiou's cult at the time, and it gets worse in one rather pointless episode in season 2. Judai and Sho were just minding their own business when White!Asuka splashes them with a bucket of water. I know that being in Saiou's cult means that someone has committed a Face-Heel Turn against their will, but what Asuka did is just an unnecessary Kick the Dog moment.
cypsiman2: Episode 40 of Yu-Gi-Oh! 5Ds irreparably taints the series with the same sexism as its immediate predecessor. Aki Izayoi, up to this point, was an intensely badass and deeply troubled young woman, and as such the topic of her past, which was merely hinted at, was an intriguing one ...until it is actually revealed. Her backstory is mind-numbingly trite, cliche, wangsty, and melodramatic. It is impossible to take Aki seriously after it is revealed, and it also serves as the turning point for her entire character, in which everything the least bit interesting, awesome, or even relevant to the plot is stripped away. Yugioh 5Ds, for ruining one of the best characters ever conceived for the franchise, I have two words; you, fail.
Valkir: Despite featuring the above character in a fairly positive light, the finale to the Team Unicorn duel tainted 5Ds beyond all recognition. Yusei is down to no cards in his deck and all Jean needs to do is end his turn for a win, delivering the first true loss to an otherwise Invincible Hero. All this time, Jean's team has been all about using team strategy for the good of everyone. But Jean is suddenly...er...passionate about his duel with Yusei and suddenly feels the need to attack him, which his team inexplicably supports. He runs into a defensive monster that Yusei boosts with Fortress Warrior and Yusei pulls away with a win despite deserving to lose in every single way. We're supposed to believe that team victory between three people that are extremely cohesive and genuinely care about one another is less important than one superstar flexing for his friends, despite that the former resonates with the themes of the franchise far more than the latter. Yusei's plot armor actually hypnotized Jean into losing. "End your damn turn, Jean" has become synonymous with "jumping the shark" in the fandom. The worst part? It wasn't singles elimination, so Yusei's team had no story requirement to win at all...they could've won their next match and still stayed in the plot.
Drake Clawfang: To be fair on that point, it wasn't merely that Jean is suddenly passionate. In a detail lost in the dub, Jean was worried Yusei would have a reversal ready to somehow stop his loss from deck-out, in which case Jean was completely screwed. Given that just a couple turns earlier Yusei had pulled out a Fusion monster, not to mention all the other tricks he's capable of, the fear that he had some sort of magical "skip your Draw Phase/place this card on top of your deck before you draw" card in his Graveyard that would delay his loss is understandable. Heck, cards with the effect of "You can add this card in the Graveyard to your hand during the Draw Phase instead of drawing" are not entirely uncommon.
Tropers/A.vonZiggurat: One astute fan pointed out that there was only one card in the entire game that could have saved Yusei at that point: Meteor Flare, which Jean had no reason to think Yusei would include. Of course, considering that Yusei had become completely invincible at this point, who's to say he couldn't just make something up?
cypsiman2: I decided to be generous and give Yu-Gi-Oh! ZEXAL ten episodes to sell me on it, and wouldn't you know it, episode 10 was the most horribly awful thing ever. To summarize: Yuma's little friends point out how he's only been winning lately because of Astral and the Numbers cards, so he declares that he'll go after Shark and beat him without Astral or the Numbers. He then finds out that Shark has fallen in with a rough crowd since they'd last met, and has even given up dueling. Yuma then declares that he wants to help Shark because they are friends and their duel gave them a special bond, despite the fact that that wasn't why he was looking for him in the first place, or the fact that he hadn't spared Shark even a second thought in the preceding 7 episodes. He then harasses Shark into accepting his challenge, even offering up The Emperor's Key despite how precious it is to him. He also repeats that he will not use any Numbers cards, as that would be grossly unfair. One turn into the duel, and Yuma already breaks his word and summons a Numbers anyway just so that he won't lose. Which he does anyway! And just to rub it in, Astral points out after the fact what Yuma could have done differently to have at least avoided losing so badly, albeit it would still involve using a Numbers card. In short, this episode demonstrates that Yuma is a disingenuous, dishonest, lying, cheating, promise-breaking little rat lacking in any sort of merit to balance any of that out whatsoever. Why I would be the least bit interested in following this kid's story, I've no idea.