The Yu-Gi-Oh! anime is a great time, but one of its main problems is that it features a ton of filler arcs. Almost half of the show is composed of filler arcs, which means half the show is inconsequential to the plot. All of these arcs also come with a lot of brand-new, exclusive characters.
Some of these characters, like Rebecca Hawkins, are a bit more middling, with some good aspects, and some bad. Others, though, are either really enjoyable or just not a good time at all.
Great: Zigfried
Zigfried is a pretty fun villain overall. After all of the world-ending plots, he really just wants to destroy Kaiba because he's always No. 2. He's completely unhinged and will resort to cheating if he has to. In addition, his big showdown with Kaiba is an absolutely fantastic duel that is incredibly fun to watch go down. Zigfried might not be amazing, but he's at the very least a ton of fun to watch lose his mind.
Bad: Valon
Valon is frustrating, to say the least. His armor cards make no sense and are designed specifically to let him punch his opponents in the face for some reason, his accent is obviously fake and incredibly annoying, and his attitude is grating. He's just not a villain that is enjoyable to watch. His greatest aspect in the end is getting to watch Joey trounce him in their duel together. Him losing is fun, at the very least.
Great: Rafael
Rafael is the better villain from the Waking the Dragons arc. He beats Atem in a shocking upset and forces him to confront the evils in his heart. No one is perfect, even Yugi. He forces him into a corner and causes him to use the Seal of Orichalcos.
All-in-all, Rafael is interesting, and he has an actual arc through the season, which is more than can be said for Valon. He's not one-note, which is what happens with a lot of villains throughout the filler arc, such as the memorable character Yami Alexander.
Bad: Yami Alexander
Yami Alexander is the most boring villain from the most unmemorable arc. Capsule Monsters is hard to get through despite only being a few episodes. Yami Alexander is the villain, being the dark side of Alexander the Great split apart by the Millennium Ring. He has little personality and is a caricature at best. He is probably the worst villain in the entire series, and maybe the worst character in general.
Great: Noah
Noah was a good villain because he fit his role perfectly. He was a bratty child who thought he was better than Kaiba. Despite this, he threw a fit when he didn't get what he wanted, and he lived in a world that bent to his every whim. While his arc would have been a lot better if his final fight had been with Kaiba, overall Noah is a solid villain who viewers simply love to hate.
Bad: Almost Everyone From The KC Grand Championship
The KC Grand Championship arc was full of a bunch of duelists that could have actually been interesting. None of them get any development at all. They're all caricatures of one aspect of their personality. Save for people like Vivian Wong, who actually get a bit of a storyline, they're all very irrelevant.
Considering that this season introduced so many new characters in a single arc, it was just a bit surprising that absolutely none of them were utilized in any real way. At least the important characters were developed, like Leon for example.
Great: Leon
Leon was certainly a fun character to watch during this arc of the show. The show makes it pretty apparent that he is an important character, but fans aren't really clued into how until the very end. The "brother" reveal at the end of the arc is pretty hard to see coming except for a few key indicators. In addition, Leon is also complex and has a good personality that is fun to watch be explored throughout the tournament.
Bad: Alister
Alister's motives for hating Kaiba are silly. He hates Gozaburo for what he did to his family and country (which of course was an illusion created by Dartz), but Kaiba undid all of that. He turned a weapons company into a gaming company. Shouldn't Alister be happy that Kaiba did this? His motives don't make any sense, but he certainly knows how to rock a crop top, so he's got that going for him.
Great: Dartz
Dartz is a pretty solid villain all around. The Waking the Dragons arc is messy, but one thing that it does well is portray Dartz as a pretty powerful force. Even when it comes to the stocks that he owns in companies around the world. Viewers can feel that he is pretty powerful, and it's a good build-up, even if he does fall somewhat flat near the end of the season.
Bad: The Big Five
The Big Five are sort of like better versions of the KC Grand Championship duelists. They're pretty basic characters with little development. Their concept is smart as a whole, but they really aren't handled right in the long run. All in all, they could have been much better characters.