Portgas D. Ace (One Piece), Hiei (Yu Yu Hakusho), Killua Zoldyck (Hunter x Hunter) and Momotaro Tsurugi (Sakigake!! Otokojuku) will be playable characters, while Heihachi Edashima (Sakigake!! Otokojuku) will be a support character.
And it's amusing to note that Heihachi Edashima and Tekken's Heihachi Mishima don't just share the same first name (and somewhat similar surnames, at that); they also share the same voice actors (Daisuke Gouri and Unsho Ishizuka).