Board Thread:Sonic Boom/@comment-27749615-20160223005021/@comment-477304-20160224040116

Sonic CD was an exclusive title for the Sega Mega-CD attachment, that is why it was not called Sonic 3, it would be unfair to those who did not have the attachment. As such, her fist non-exclusive, non-spinoff title in the series was Sonic Adventure, by which point Knuckles had already been introduced. This is the most likely reason why Amy often appears after Knuckles in character select screens.