[Matrixbeast]

I just wish Gills was more relevant to Agito as a whole. The guy goes through massive swaths of episodes without even interacting with the other main cast members.

Anyway, whoever is tertiary doesn't really matter. It's not like Toei is chomping at the bit to do a "Tertiary Rider Ultimate Team Up!" special. Outsiders is bringing back villain/antagonistic riders. Girls Remix brought back a swath of female riders.
The classification of tertiary doesn't really hold much ground beyond trying to decide which character has marginally more importance to the plot. Often times, such as this case, it's difficult to say. Na-Go may suffer more, but Michinaga's actions tend to directly alter the plot's trajectory.