Riley is a tough as nails survivor, which I think is important when you have a group of people who are being hunted to death. Plus she's the only one who's ever known what it's like to be a parent
even if it only lasted a night
. That's pretty important.
Besides, she initiated the "Hey, What's Going On?" scene, which is basically the best musical scene in the history of television.
Will — sense of justice, righteousness
Nomi — social justice
Lito — acting, "lying for a living"
Sun — fighting, loyalty to family
Kala — chemistry, religiosity, emotive
Wolfgang — fighting, surviving; not as "noble" (stealing, murdering, etc)
Capheus — surviving, transportation, protective
Riley — surviving, artistic soul
I also think that Nomi, Sun, and Will have more logical mindsets, while Riley and Lito are more artistic. Kala and Wolfgang's relationship is interesting because one is very moral, and the other is painted as being much less so.