I think Keiko was a good buffer for O'Brien- yes, not as joyful and carefree- that could be racial, but she keep him from being to xenophobic too, insisting he give Cardassians etc. a chance- like the young boy- that would have gone totally different without her.
Um...you mean Keiko's behavior was racial? I don't think I buy that...


But I think the same purpose could've been accomplished--putting O'Brien on a more even keel--without her having to be a jerk.