i wonder why Nero should be the character in DMC 4 i completed DMC3 and i love it dante *bleep* up Vergils ass. and manage to keep the gateway close..nero has nothing to do with dante bcause he is in good side ..what is the connection between two?????????????????????????:i thought this game will be greater then DMC3

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Nero is the main protagonist in DMC4 because a consistant use of Dante will eventually make the series stale. We already know mostly all information reguarding Dante; even to the point where he can defeat Mundus, the most powerful demon.
Nero can help expand Dante's story further; along with bringing newer fans, also incorportating Vergil too for the DMC3 fans.
The connection yet is unknown; although there are many theories, Dante and Nero are connected though; Dante seem's to watch over Nero consistantly all throughout.

Here's a quick screen cap that not many noticed:


Check out Dante watching him.