I think the idea behind Kenzaki for most of the first half is that they took the "goody-goody heroes aren't interesting" approach, but subverted it by also building towards the later, darker storyline of Kenzaki.
He starts out as just the nice guy who lost his parents and signed on to do some good. In the first half he's like the beacon of sanity in a world gone mad. The only people he can trust can't do much more than set him off in the right direction. His workplace is trashed, his boss is missing. Tachibana's jumping around from being shifty, mind-controlled or seriously contemplating a career change. There's that whole "Hulking out" thing they worry about early on, which seems like it's the result of someone thinking that people dying from turning into Kaixa was the coolest idea Rider's ever had, ever.
Hajime's pretty hard to read too. Amane gives him grounding so he's not quite a "villain", but early on I was sure he'd not survive the end of the show. Which is maybe why Missing Ace feels like the ending to the show at the time it was made (pre-30-something-ish), rather than when it's meant to be set. For the first half, Hajime's a sympathetic anti-hero, but I could see him being the one who makes the sacrifice in the end to save the show's namesake. That's just how I thought at the time though. Agito wouldn't have ended with Shouchi taking the headbutt so Kino could open up a private clinic.
While by the end, Hajime becomes the only one Kenzaki can really rely on should things get that bad, and vice-versa. They're practically brothers. The final battle is a no-win situation. If Kenzaki seals him, Hajime's gone and we get a nightmarish world of a Missing Ace Riders and Orphenoch cameos up the wazoo (though the ultimate ending is a happy enough one.) If Kenzaki throws it, well, we see what happened. And while I love both endings, I gotta say that as Kenzaki fan, that TV one hurts. It's arguably triumphant for Kenzaki though, in a twisted way. He loses out, but he saved someone else, and that's really all he ever wants.
I like to cite that big rumble in episode #7/8 where we get our first glimpse that Kenzaki's got more under his skin than we're originally meant to think. But for a lot of the first half, yeah, he's mostly Mr. Hero since everybody else around him is constantly going back and forth on the good/bad scale. It reminds me of Ryuki, in a way. At first glance, Shinji is probably the least-interesting of the Riders. He doesn't have a comatose girlfriend, isn't dying, isn't out for revenge or has some loopy grand scheme.
To the others, he's a naive fool who just doesn't get how the real world is (which they really stress in the TV Special with Verde and his dog-eat-dog philosophy.) But in truth he's more badass than anybody, because as idealistic as his goals are, he's also the one who's ultimately right, and most of the time, he sticks to his guns. I like some of the others more on a design/powers basis, but Ryuki's easily my favorite character-wise from the show.
I do agree that in the end, Blade's as much about Hajime as Kenzaki, if not more. Again like in Ryuki, where it really starts to focus on the importance of Yui. Shinji and Ren are the main men, but everything hinges on Yui. In Blade, it's Hajime that's on everybody's minds, especially later on. Like I said, the final battle is a no-win situation for the characters, and presumably the audience. By that point, we're invested in both of these guys' fates, and as cool as it is to see them kicking each other's asses across the woods, we don't want either one to lose, because they both deserve happiness.
But it's still very much about Kenzaki. It's about his fall, actually. The series takes the most heroic and dependable of the four Riders and puts him through the gauntlet, drags him down, ultimately "killing" him a way, though you can also read it as his ultimate victory. He saves Hajime at the price of his own humanity, spitting in the face of fate and giving life to someone who deserves it. But he's Kenzaki, so he would do that, wouldn't he?