After Allen Walker turned to the age of fifteen, he surpassed synchronization with his Innocence but, what came later was truly unexpected. Which caused Leverrie and the Black Order to denounce Allen a heretic that must be destroyed.
From the hatred of Innocence after escaping from the Black Order with Cross, the noah within him evolved. It became a dangerous weapon, from time to time it would take over his consciousness, and that noah called himself "Neah", everytime Neah destroyed Innocence, Crown Clown's synchronization would increase by 2%. When Allen came across Innocence, Neah would come out and at no cost, annihilate the Innocence's very existence.
"Allen...Go to Cow's sigh and buy me some liquor". Cross demanded.
"No way !" Allen said. "Why should I? You can go and work for the money to buy it, master." And so they argued, which brought forth Neah out of Allen so Cross can party!
Some six years after the incident Allen rejoined the Black Order again after Leverrie's death in an Akuma invasion. Lavi married Miranda, Aleistar Krowly was still wishing for Elliade, Kanda as the usual calling "Allen" beansprout, and Komui the head supervisor went missing after Lenalee's death. Everything seemed fine but it wasn't. The Allen that everyone knew was gone. In his place was Neah the Fourteenth noah, but none knew what was ahead.