Didn't Project X Zone divulge that Lady is more than human because she has some priestess blood in her or something? Either way
I don't think there's really much of a difference between the humans in either universe, given that Lady couldn't take out anything tougher than a basic goon (albeit with the requisite style).
But demons can only be killed in Limbo, right? Mundus was exception cause he drew power from the Hell Gate so as Vergil says, in his case, if they kill his human body, they kill the demon too once the Gate is shut..
Demons are powerless in human form because they're soul is locked in the body with them. That's why Limbo is so important to them, it's the place where their powers can affect the physical realm. In Limbo they can still be killed, because they're in their true form (complete with their soul, again). It just so happens that in Limbo, you're litrally on their turf, and killing them is gonna be much more difficult.
Mundus was immortal because he kept his soul past the Hellgate, which he also used for power. Their plot to draw Mundus away from and close the Hellgate was because it shoved his soul into his body, where they could kill him.
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Kat is very much a pure noncombatant. It's just not what she was trained for, or is capable of without extensive training. Whatever other spells she may have, they do still take time to prepare (she's said as such), and at most when The Order HQ was being raided, she certainly didn't have more on her than what allowed for bridging to Limbo. She did what she could, and (directed at only a few obvious people) who gives two curly brown wads about how useful she is compared to Lady? They're interesting characters, although I find Kat to be a bit more unique in concept and execution.
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And I'm sorry guys, I gotta...
No. Because she blindly followed his cause. Even when he abandoned her she didn't even question if Vergil gives a damn about her. She is the most obvious example when somebody asks you to jump from the roof and you jump, without even thinking.
Kat did not blindly follow Vergil. He showed her that everything she was seeing was real, and the world was in fact in quite a lot of trouble. She followed him because she had the ability to help. Sure she admires the guy for having saved her life and opening her eyes, but she did not just blindly go along with whatever he was doing.
And "even when he abandoned her she didn't question if Vergil gives a damn about her"? You mean twenty seconds before the credits rolled, where no more dialogue could pop up. Or maybe she didn't question him because he flat out called her simply "useful" and nothing more during all his talk about how weak and dumb humans are. She didn't question Vergil's departure because a) the guy straight up dissed her and her race, and b) if she truly had been blindly following him, she would have been way more concerned about his disappearance, but no, her commitment was to the cause, to Dante, and to humanity -
not Vergil.
And seriously dude,
seriously, just
stop. You've been proven factually wrong countless times over a multitude of different circumstances. Your slanted opinions on DmC just don't hold up against people who actually know what they're talking about.