Vergil explaining what the demons are doing in the human world early in the game:
Vergil: The way they keep us (humanity) docile is a... a little more insidious.
Dante: What is it? *sips drink from can*
Vergil: It's in your hand. Planet Earth's most popular soft drink, Virillity. The demons have spiked it.
*Dante slowly turns and spits the drink out*
During the mission to break into the Virillity Factory to find Poison,Dante questions Kat on how she met Vergil and she tells him that they met in Limbo. Dante then asks why she was there to which she responds that she was escaping the nightmares. Kat's reply gives us Dante's cutest WTF face ever.
Dante: You came to Limbo to get away from the nightmares? Kat: Yes. (Dante silently mouthes "fuck me" when Kat walks away)
Prior to Bob Barbas's boss fight, Dante gives him the middle finger and challenges him to "put a spin on this". And he does.
During the boss fight with Bob Barbas, you can see a bit on the newsticker that reads:
Newsticker: "Bob Barbas voted the world sexiest news reader."
A nice Oh Crap moment for the player when the three are invading Mundus's building. As Dante heads up, he tries to remember which floor to stop on, thinking its 105. We then hear Kat going over the plan telling him to stop on 106 and not 105 as it's infested with demons. Needless to say, thanks to his mistake, you now have a whole floor of opposition to get through.