Tough question. I really like Natsumi (wait, a female Rider who gets to live?!). She does play some of the emotional support/moral guide role in Decade, but to be fair so does pretty much everyone. Tsukasa needs as many Jiminy Crickets as he can get.
Ozawa is made of awesome. Smart, forceful, resourceful. What's not to like?
All of the Kuuga supporting characters were wonderful. They felt like real people, competent at their jobs, handling an outrageous and horrifying situation in the best way they knew how. Good female characters, good foreign characters, and an obvious firm belief that people are fundamentally good... throw in some actual competent medical knowledge (for the most part) and oh do I love this show.
I have mixed feelings about Akiko. I wish they had given her a few more scenes where she got to play things as actually serious. Yes, everyone was over-the-top a lot of times in Double, but all the male leads got a fair smattering of times when they got to play it straight. If they had done that with Akiko--given her a bit more development about what she's trying to do in Fuuto with the detective agency, given some more development between her and her father, given her some time to grieve--she could have been awesome. As she is, I just feel like she got shafted in the character development department. A lot of potential, yes, but not the follow-through I would want in an awesome female lead.
I really do hope more is done with Hina in OOOs. And though I'm pretty sure there won't be, I keep hoping we'll get some explanation for the super-strength.
Really, we just need more female Riders. Then they wouldn't have to try so desperately hard to make the female leads on par with the male leads in some other way. At least Double acknowledged that girls want to be the Rider, too, in their dream episode.