User blog comment:GreninjaBoss11/Adventure Mode Bosses and Stages/@comment-28616980-20160816060029/@comment-25096445-20160816135448

Your assumption with how it progresses is correct. You play through a character's stage, and it unlocks several others to create an illusion of a branching path. Also, I've never played Pikmin, so making Oliver's boss a Bulborb was an uninformed mistake on my part. Will change it soon.