The human cast of Sonic X were never going to make anyone's top ten list, they seem so out of place next to Sonic and his friends and even Robotnik who actually is human but there is one human character who is not only a good character but is actually the perfect foil for Rouge the Bat and that is Topaz.

Originally designed as a catalyst for Rouge joining the GUN organisation, Topaz can be best described as the straight woman to the antics of her partner Rouge, the squeaky clean professional against the teasing cat burgler persona of Rouge, in fact their first meeting, Rouge teased Topaz about being in to S&M, (Japanese only) from there they become inseparable partners throughout the series.
I've always been into female duos, the character dynamics between Rouge and Topaz are a perfect yin and yang, I just wish that the games realized this and give Rouge a proper partner, although her role with Shadow and Omega is just as effective.
I'm still very critical of Sonic X and with good reason but I can appreciate when they do something different that's actually decent and works really well.