Most well developed character in the original series?

[Captain Cadaver]

*Note that by "Original Series", I'm referring to the original manga and the Duel Monsters anime, not :toei's "Season 0".

As far as character development goes, I wouldn't say YGO has any that stand out. Yugi had some in the last arc, but it was quite rushed after Atem had been carrying him for about 90% of the series. Kaiba going into the role of disgruntled rival/anti-hero after having been a villain remained static after the Duelist Kingdom Arc. Overall, I'd probably say Joey/Jonouchi since he had some development in how he changed his strategies. Even if his luck based tactics in Duelist Kingdom and early Battle City were amateurish, the series at least did a decent job at portraying him becoming gradually more competent as time went on, especially by the time he duelled Marik.

 

[Papasmurf]

Probably Jonouchi as well. Kaiba's development is actually closer to a character rewrite if you count the first 7 volumes, in which he's an irredeemable psychopath and a murderer, who had no honor whatsoever. Yet Atem, simply because he had 2 duels with him, suddenly starts prattling about Kaibas honor when some Pegasus goon uses his cards (which werent even his originally; he stole all 3 BEWDs)...

Yugis on the other hand is decent if you compare his chapter 1 self to the guy that beat Bakura and ATM, but it was far too concentrated in the final arc to be great character writing. He should've fought the non-finals duels himself in Battle City instead of relying on Atem.

 

[withheldforprivacy]

Probably Jonouchi as well. Kaiba's development is actually closer to a character rewrite if you count the first 7 volumes, in which he's an irredeemable psychopath and a murderer, who had no honor whatsoever. Yet Atem, simply because he had 2 duels with him, suddenly starts prattling about Kaibas honor when some Pegasus goon uses his cards (which werent even his originally; he stole all 3 BEWDs)...

Yugis on the other hand is decent if you compare his chapter 1 self to the guy that beat Bakura and ATM, but it was far too concentrated in the final arc to be great character writing. He should've fought the non-finals duels himself in Battle City instead of relying on Atem.

The goon who stole Kaiba's cards was the technical winner against Yugi, something the real Kaiba never managed...