you can`t.
Chat with our AI personalities
Sonic will appear during the final battle in the Subspace Emissary
Beat the subspace emissary.
Those are stickers, and they're used to power up your characters in the Subspace Emissary. You can find them randomly in the Subspace Emissary, and they also appear in versus matches.
Having him join you in subspace emissary is the easiest and fastest way.
Kill all the boses