[Kurona]

Whatever works for the series, really. G3-X, Chalice, Cross-Z and Birth are all some of my favourite Secondary Riders for very very different reasons.

But if you asked me to pick, I would say that I have a soft spot for the 'classic, traditional' way of doing it -- introduce a new character about a third into the show. W, OOO and Fourze all stuck by this; and they're very formula driven shows that KNOW what they're doing with that formula and made it work. Accel, Birth and Meteor all stick out in my mind for a reason.

Though also, like, one of my favourite Kamen Rider characters ever is Banjou Cross-Z? And I kind of love "character introduced early on through their development eventually gets the power to become a Kamen Rider".