[LockonStratos]

Honestly, that would have to be a character like Hibiki or Tendou. Someone who's skills and life are basically already there and don't really have character development so much as revelation.

It also basically requires you to have the younger, less experienced, secondary rider introduced far sooner than usual, even without them actually transforming. Again, look at Kabuto and Hibiki, with the assumption that Asumu was meant to be an Oni by the end.