According to Gamespot: Sonic joins previously confirmed series mainstays like Princess Peach, Mario, Luigi, Donkey Kong, Link, Toon Link, and Samus, as well as newcomers Mega-Man, Wii Fit Trainer, and Villager.
Super Smash Bros. for the Wii U and Super Smash Bros. for the 3DS are . There will be no cross-platform play between the two. Now I remember how happy I was to see Sonic in Super Smash Bros. Brawl back in 2007, when he was originally meant for Melee. But I'm totally happy to see that the is coming back. Back in SSBB, the non-Nintendo characters were both Sonic and Solid Snake (Metal Gear/Metal Gear Solid). Snake was an awesome addition to the game and he was cool in terms of gameplay. Not to forget, his Codec calls were all funny.
For more info on the new Smash Bros game check out GameSpot's interview with director Masahiro Sakurai.
God Bless and Play Hard Gamers!