I know but the criteria can be a bit screwy at times. Especially since every other series is pretty clear about who's the secondary.

Well, according to Toei, Blade's secondary is Garren. That's why DiEnd uses Garren, not Chalice.

Diend uses everyone.

I don't think that's a logical answer as to why Garren would be the secondary rider.

The question is: Is there a logical answer in a show with four riders which are all treated about equally in status? I mean, I don't think the show had the concept of "secondary rider" during production (the definition of Secondary riders was actually not that clear until we had shows with just two riders, like Den-O), so technically Garren and Chalice could both be considered the secondary. As for Leangle, I guess he's not important enough.
But how do we define secondary in a multi-rider show?
Also, adding to the confusion: DiEnd uses both Chalice and Garren. Haven't seen that show in a while so I had to check.

List of official secondary Riders

http://i.imgur.com/5Snrquh.jpg

And consulting my handy katakana cheat sheet, Blade's seconday Rider is Karisu/Chalice.

Guess that settles that.

Where is that list from? A magazine? A book? If we had the full thing, that may help answer a lot of question on other topics.

In Kiva why does Nago only use the whistle to summon the sword from kiva once and then just ignore the other two whistles?