Multiple characters but they have to have a gimick that changes the way you approach levels in a meaningful way, which Tails, Knuckles, and Ray all do. Metal and Shadow however, do not.

Whenever I say this Shadow and Metal fanboys come out of the woodwork with uninspired gimmicks to justify their inclusion, instead of coming up with meaningful mechanics first and then making characters to reflect those mechanics. Knuckles, Tails, and Ray all approach the levels differently enough that you'll take different routes and add replay value. Every implementation of Shadow and Metal I've ever seen play about as different as Sonic in CD vs 3 (small variations).