Ashura Douji. Oda built him up tremendously early on in Wano. He was introduced as this strong yet disillusioned Samurai warrior, and it made you wonder what could've led him astray. Oda dragged his plot line on until it was abruptly concluded, and he fell into the background as just one of the Scabbards.
He was also a convenience machine, stocking the Alliance with everything they needed out of the blue, like the mansion blueprints, warriors to fight, and ships. That wouldn't be as bad if he were introduced still having hope, but he wasn't. If you had all of that laying around, why did you brush off everyone thinking there was no point in fighting?
Easily one of the weakest Scabbards in terms of characterization.