Thread:Digi/@comment-25083716-20140620042408/@comment-386913-20140620123824

The rules behind who the secondary Rider is generally change between eras and periods and such. In the Heisei era, the secondary Rider generally shows up early and partners/associates with the main Rider early on (G3/G3-X, Knight, Garren). There may be other if not more important Riders who appear but they're generally seen as the initial partners. Really the "second Rider" stuff didn't change until about Kabuto, where Gatack was seen from the beginning but didn't get his form until later which happened to be another beetle, thus complementing Kabuto better than the other bug Riders regardless of their story. And really by Den-O it solidified with Zeronos essentially introduced as the second Rider. The only exception to the Kabuto-style second Rider is Baron, mostly due to the early-Heisei style and rules making him the "second Banana" to Gaim.