I myself would choose Roll Casket from the Megaman Legend series. True, there isn't heavy character development like XHunter had mentioned but still this Roll had quite the interaction. It was also cool that she was the top mechanic for Megaman's armor, chips, headgear, footwear, and of course weaponry.
All in all, character development aside I liked seeing the slightly tomboyish but witty Roll go to work on and off the field during the Megaman Legend games. It is true that the original Roll was the ground base but it doesn't mean that Roll Casket wasn't another base for Roll.EXE. Game designers always have to work with something, y'know, especially for their characters.