I think I'm going to go with the "if it fits, it fits." crowd on this one. Adding however that I think there's a distinction in the realm of Kamen Rider. That being in the way of cast-size.
If you're doing something like Ryuki, where your Rider pool is very large, I think you can afford to not fine tune your protagonist quite as much, at least in the beginning, as he or she will have many foils and contrasts and other interactions with the rest of the Riders and cast. I think you can afford to characterize your protagonist a bit less because in a way it helps to act as a mirror to show off the rest of your cast to their full potential. Ryuki did this well I think by putting Shinji's bigger developments towards the end, after everyone else had had a chance to be highlighted. He still experienced significant growth along the way, but his personality still helped to show off the other people around him well.
If you're doing a smaller Rider pool? I think a more developed, more well-rounded and otherwise all-around personality is required. You don't have the sheer bodycount to help personify the concept of say, a psychotic Rider, so the next best thing is to maybe make your protagonist go berserk. *pours out a bowl of OOOs into his cereal bowl*
Okay, that was a bad joke. Gimmie a break. You know you eat the marshmallow Ankhs right out of the box 'cause they're so tasty. That's why I never get any!
*cough* anyway....I think I've rambled enough. Bottom line is that I think it depends on your cast and how big it is. A larger bodycount can allow you different freedoms in character creation than a smaller one.