In many regards, Nei and Rika are polar opposites.
I adore Nei, the original Numan. She is gloomy, however, with a very sad tale. We also don't have her for very long in the game.
Rika is cute but can be annoying at times, I'll agree. However, she is extremely childlike as she is inexperienced to the real world. I excuse Rika's halfbrained statements for that reason; of course she is going to seem incredibly naive. Give her some credit - for a one year old, she is really, really intelligent.
It's a toughy, but Rika gets my vote.