The multiplayer characters in sonic the hedgehog next generation are:

Sonic the Hedgehog

Sliver the Hedgehog

Shadow the Hedgehog

And that's it, nobody else.

You can't :3 You can play as Sonic the Hedgehog and Blaze the Cat, but no other characters are playable.

He only did the voice of Sonic.

Sonic the Hedgehog is the name of the official characters and series whereas Sonic Boom the Hedgehog is most likely the name of a fan character.

You can be Sonic the hedgehog or Blaze the cat.