I'm no Tekken expert or anything, but I've noticed that Heihachi is usually the "cross-over" character from that series. I always thought that Jin was the protagonist? I know that some companies do not promote the protagonist as much as others, but what role does Heihachi play? I know he's Jin's grandfather or something like that. Is he the antagonist?
Heihachi is the main antagonist of the series. He's essentially the M. Bison of the games, though there's usually "true" final boss after him, like Ogre or Unknown.
The protagonist of the series was originally Kazuya (Heihachi's son), though in
Tekken 3, Jin (Heihachi's grandson) took over. The central theme of the story is the war between Heihachi's organisation, the Mishima Zaibatsu, and Kazuya's organisation, the Tekken Corporation, with Jin caught in the middle between the two. Heihachi tries to use Jin's cursed "Devil Gene" to awaken the boy's full power and take over the world, while Kazuya tries to use Jin for his own purposes and ultimately defeat his hated father.
Heihachi is just a very iconic character and essentially the mascot of the series, in the same way that Nightmare is the antagonist and mascot of the
Soul series, even though Siegfried is the protagonist for most of the series up until
V, when Patroklos takes over.
tl;dr - He's the bad guy, but he's very popular and the most iconic character in the series.