[Malunis]

A great point I saw for the show is that each saga kind of sets up a different character for the secondary Rider role.

Episodes 1-11 is about Ryugen as Gaim's go-to guy and a slow look at the secretive side to his character. Episodes 12-19 have some focus in which Kaito becomes less of an enemy and more of an ally, but on temporary terms.

And while we're still in the 20-onward saga, there has been a lot of Takatora and a lot of explaining what his deal is and how he and Kota CAN get along, were nothing stopping them. By now, all of these guys have a story going on at high speed, which makes it exciting to see who takes the role first.

But going by "who appeared first" rules, technically Baron would definitely be classified as Gaim's true secondary, he meets the "gotta antagonize the hero" requirements as well and makes for GREAT contrast. And while Zangetsu appears an episode earlier, the official site doesn't really seem to officially acknowledge him until episode 4.

Plus like it's been said here, they definitely market Baron under that role a lot. It always struck me that whenever you see promotional material for the movies, you hardly see anyone besides Gaim and Baron, to the point I've had friends ask me "Are any of them even in the movie?"