Cool arm? You mean that spam blaster that did a major amount of damage on all enemies including bosses?New Dante is Nero but dirty and with bad manners (and no cool arm)
Not only that, the supposed "cool" arm is never explained or brought up.
I'm not entirely sure if you've played DmC yet. Though this newer Dante has some of Nero's combos from Devil May Cry 4, that does not make them even remotely the same character.
New Dante is a young adult who lived a tough life and had a rough childhood. He didn't remember his family, had no friends, and was treated like garbage most his life. This relates to why he despises society so dearly.
He drinks, has no ambition, and doesn't seem to care about humanity until Vergil and Kat find him.
Nero... well... we know nothing of his childhood or background.
He seems to be very protective of his girlfriend, and has a very similar cocky attitude to old Dante.
That's it.