How to Unlock Sonic in Super Smash Bros. Ultimate
So you want to go fast? SEGA's speedster once again returns to this iconic fighting franchise in Super smash Bros. ultimate. Like the majority of the cast, Sonic is actually not available from the start. However, there are multiple methods for obtaining this fighter in-game and he is one of the earlier combatants you can acquire.
The first and most consistent method is by completing Classic Mode once with Mario. Sonic is guaranteed to appear if you beat Classic Mode on any difficulty, however, you will have to defeat him in a 1 vs 1 scenario. He's a pretty easy combatant to defeat, especially with Mario since you can bully him off the stage with your hits.
If you don't want to play Classic Mode, Sonic is the 17th fighter to appear naturally by just playing Super Smash Bros. Ultimate games. This is a slower method, but you'll unlock a ton of characters before reaching the blue speedster.
Additionally, Sonic is not unlockable in World of Light, so if you really want this fighter just do one Classic Mode with Mario. It's far easier than just grinding out matches since you'll only have to do around 7 to actually obtain him. Once you defeat him in combat Sonic will be instantly and permanently added to your Super Smash Bros. Ultimate roster.