Instructions
Play 300 matches or play for 10 hours in Versus Mode. This is the simplest way to unlock a match with Sonic. It also takes the longest amount of time. If you do not defeat Sonic, he will not be added to your roster. If you defeat him, Shadow will be added to your trophies.
Clear Classic Mode with 10 different characters. This is slightly more difficult because it requires you to win. Select a character and fight your way through all of Classic Mode. Then do it with nine other characters. After you̵7;ve beaten Classic Mode with 10 characters, Sonic will challenge you. Defeat him and he will be added to your roster, and Shadow will be added to your trophies.
Beat Subspace Emissary. This is very challenging, but completing it will unlock Sonic and several other characters. During the course of this mode, Sonic will challenge you to a fight. Defeat him and you will unlock him as well as the Shadow trophy.