For all of his grandiose entrance and efforts to reach Gai and Inori before it's too late.... well, it's too late by the time he reaches Gai and Inori.
Thus, Mana is reborn as per the plan of Gai and company, and although she's thrilled to see Shu the feeling is anything but mutual. The reason for this soon becomes clear enough, as Mana undertakes a bit of ballet - not, in itself, the most horrific thing ever imparted upon mankind (that would be the Riverdance), but on this occasion it is the catalyst for the Fourth Apocalypse, turning vast swathes of the world to crystal while Gai shows his strength in beating down Shu and the rest of his gang struggle for their lives against their various opponents.
All hope isn't quite lost however, as Shu spots a lone, solitary crystal flower - the final vestige of Inori's being. Grabbing hold of it using one of his many Void powers, Shu unlocks the key to reverse Mana's effect on the world, stopping the growth of the cancerous virus and also allowing him to turn the tables on Gai into the bargain. With the world saved (and a glimpse into Gai's mindset as a bonus), it's time to finish the job and soak up the "sin" of the virus and Voids across the planet - something which Shu and Inori achieve together, although the latter has one final trick up her sleeve to save her beloved from his fate.
So ends Guilty Crown, a series forever tainted by its place in the noitaminA programming block to which it was rarely, if ever, suited - seeing this series presented in a slot for off-beat, thoughtful and decidedly different anime was rather akin to going to an independent theatre for highbrow, low-budget films and finding that they're screening the latest Michael Bay movie. This is, to some degree, hugely unfortunate - Guilty Crown offered plenty of popcorn entertainment and spectacular action sequences that felt designed for a first or third-person shooter video game rather than an anime, set against a smart concept that never really had suitably intelligent writing to back it up. It's for that reason that Guilty Crown has, for the most part, been my guilty pleasure of recent months - for all its dumb moments and plot holes you could drive a mecha through (and my goodness there was plenty of them), the series was entertaining for the most part. The trouble is, it didn't entertain us in the way we expected it to engage us, and in those terms it will forever be remembered as a poorly scheduled, unfortunate failure.