Is Mephiles a playable character in Sonic the Hedgehog

No, he is not. He does indeed appear here and there in parts of the game, but you cannot play him like with Sonic, Shadow, or Silver. He is merely a background character that is a Shadow copy, like with Sonic and Scourge, and will not appear in any future games as far as we know- Meph will only appear in Sonic 360, or Sonic the Hedgehog, so that is it.

He is only a character SEGA and Sonic Team added, to give the story a little more depth... nothing more.