Follow the guidelines below to be able to play as those characters in versus mode.

Character	How to unlock
Play as a Chao	Beat all of Tails' levels with an A rank.
Play as Amy	Beat all of Sonic's levels with an A rank.
Play as Big	Get A ranks on all of Eggman's Stage.
Play as Chaos 0	Beat all of Rouge's levels with an A rank.
Play as Mecha Sonic	Beat all of Shadow's levels with an A rank.
Play as Tikal	Beat all of Knuckles' levels with an A rank.