Huh?
Wataru was a loner who slowly tried to open up, regressed rapidly, and ended up a few inches more confident than he was before.
Wataru had no confidence in himself at the beginning of the series and he he almost completed Maslow's Hierarchy of Need by the end.
Nago was an ass in the beginning and did a complete 180 by becoming a moron.
Nago's tenure as a comic relief character was only a sidestep of his development. Before that he had already shown signs of changing, he had started to care about his comrades more than his silly, self-impose, mission.
Taiga killed so many yet didn't face any consequences. (Talk about perfect morality there)
No one ever said that only good characters can get development. Taiga was introduced as being a two-faced, self-righteous prick with no regards for others. Right at the moment he though he had everything in the world, the the ground was ripped apart right beneath his feet, letting him to crawls his way back.
Otoya loved a woman 70% of the show (IIRC), then completely forgot about her after he meets another... Not only that, his character also did a 180 from becoming a happy-go-lucky playboy to a devoted dark suicidal emo dude in a few episodes.
I never said Otoya had character development. He was a the same from beginning to end, it just so happens that "same" makes for a great character all on its own.