I won't say anything about Yusei or Aki because they got screwed, but Jack did not do a sudden 180. In the Fortune Cup, Jack thought he was the best, but he wasn't because he was used by Godwin. Then Carly comes, and wants him to show that he can be a much better King then he was before, and she showed him that. And she wasn't his girlfriend. Carly helped Jack, so he could go on the right track, and I think the Jack VS Carly duel was brilliant. Now Jack isn't a lonely man, now he actually cares. Carly cared for Jack, and Jack cared for her at the end of Dark Signers. I don't want to talk about the WRPG arc because you can blame that on Yoshida, if we're talking Jack-Carly relationship.
Carly should have stayed dead after Dark Signers, along with everyone else, especially Kiryu. If he stayed dead, then we get no Crashtown arc.
If it was one person who did a sudden 180, it is Judai. He went from a loveable goof to a wannabe Atem/Yami-dude. Some other characters I think got nice character devolpment, was Chronos. He went from a hater, to a lover, and his concern for the students changed.
@Electrivire Addict: Judai VS A guy with a Question Deck wasn't filler? Judai VS a guy with ninjas wasn't filler? I do agree that Season 1 had lots of fillers though.