Disclaimer: Avatar the Last Airbender copyright Dante Dimartino, Brian Konietzko and Nickelodeon Studios.
He'd sacrificed his life.
Zuko, a mere mortal, had fought the most dangerous spirit known to mankind. In a desperate bid to save everyone he loved, he'd willingly given his life, dying alone. No one had ever known.
And he'd lost anyway.
Trapped in the Spirit World, Honor incarnate had been forced to watch as his sacrifice was wrought pointless; his spirit splintered, preventing both his and Koh's immediate death as intended. Instead, Zuko was awarded a spectral view as his enemy turned his discarded body first into a marionette and now a suit.
Zuko was powerless to stop him.