You first need to watch all previous rider installments.
The riders in Decade series are basically alternate/parallel world riders, not the original themselves.
The real Kiva is Kurenai Wataru, the first guy that Tsukasa and Natsumi met in the first episode. The real Blade is Kenzaki Kazuma, the guy who shows up in the two last episodes of the series. Kenzaki is able to transform directly to Blade King form further proved that he's the real Blade, (KR Blade spoiler, highlight at your own risk)
because he has sacrificed himself and became the Joker, in the end of Blade series.more info on original riders, please open the spoiler
» Click to show Spoiler - click again to hide... «
1. Godai Yuusuke/Kuuga = happy go lucky guy, loves travelling and likes to help people. He took the risks to become Kuuga, an ancient Warrior.
2. Tsugami Souichi/Agito = Amnesiac guy, possesses a psychic-kind of power that could turn himself into a warrior named Agito.
3. Kido Shinji/Ryuuki = A newspaper journalist, which accidentally involved in some kind of survival game. Thus, he becomes Ryuuki.
4. Inui Takumi/Faiz = A guy, forced to become Faiz after involved himself with SmartBrain. He's not a high schooler like in Decade series. Just a loner.
5. Kenzaki Kazuma/Blade = BOARD's employee, which fights the Undead. He is definately not that bright like Kendate in Decade, but pretty much similar. He then turned into that emo looking guy after some occasions in Blade series.
6. Hidaka Hitoshi/Hibiki = A cool and easy going guy. As compared to Decade's Hibiki, he's not a character that would thrown away his pride and everything else, instead he pretty much endure the risks and take them as challenges to himself. No, Asumu (nicknamed Shounen/Boy) is not Hibiki's successor, instead Kiriya Kousuke, Asumu's classmate and sworn rival is Hibiki's successor, seen at the end of the series.
7. Tendou Souji/Kabuto = Overrated cool guy. That is all I can say. Totally different from that of Decade's version.
8. Nogami Ryoutatou/Den-O = Soft-hearted and a kind guy. Not the warrior-type person. He's not a kid.
9. Kurenai Wataru/Kiva = As I have explained above. He is much like Ryoutarou, a timid and shy guy, have some sort of inferiority complex. But his character gradually changes as the story goes on.
This is just my fanboyism opinion though:
Decade series does not necessarily connected the shouwa, heisei, and post-heisei eras riders. Plot holes in the series have ruined the story essence as well as the alternate-world riders concept. I'd say the series itself is a major fanservice to its older audiences, nothing more.
i know but i thought they hire someone else to be the main riders cause they couldnt afford all the actors in decade. reason being even in den-o movie, no more adult ryutarao