I think it's once again ANGST IS DEPTH. Edgeworth, Phoenix's main rival/friend, is considered the best developed character, when you have to consider that a) he had three games, soon to be four, to get it; b) he's had at least two cases where he was personally involved (in one of them he was the suspect) which helped loads to give insight on his past; c) you get to talk to him a lot in breaks and right after court; d) I'd say half of his screentime was TEH WANGST or social awkwardness (he gets better, thankfully).

I keep saying that it's the game's fault for not having Apollo and Trucy talking with Klavier after the trials, because I've noticed it's when most prosecutors and minor characters get to shine their personalities the most in the previous trilogy. If instead of just going right to the hospital at the end to talk only with Vera we had Klavier popping in somewhere, and have them chat about how he truly felt about Kristoph (and Daryan) and going against them, these people wouldn't be able to complain about this they'd complain about something else then because he'd have more or less the same development than Franziska and Godot.

...Sorry for teal deering D: And I'd love to read you about AJ and Klavier.