Bringing Out the Blue

In this universe, the arrow injured the Blue Spirit instead of knocking him out, so Aang didn’t find out who was behind the mask. To Zuko, it seemed simple enough to maintain the disguise – just don’t take it off and pretend to be unable to speak. First of all, he needed the Avatar’s help to remove that arrow, and then he found out that the Avatar needed help himself, for his sick friends. Unwilling to leave two people to die, even for his long-sought goal, Zuko decided to use the disguise to infiltrate the Avatar’s group and find out useful information on his enemies that way. It seemed so reasonable, but then he found himself traveling with the group, and helping them out to maintain his disguise, and it all got out of his control fast.

So as far as the Gaang was concerned, Blue was someone who’d just rescued Aang and continued to be a good ally and later, friend. Zuko found such naïve trust ridiculous, but all the same, it was hard not to respond to it, or to dislike them for it. As Zuko struggled with the way that the Avatar and his companions were nothing like he expected, he began to realize that perhaps, he’d been wrong about some things – about the Avatar, about friendship, about the role of the Fire Nation in the war… Zuko believed in living by his father’s harsh rules, but he couldn’t easily apply them here. In turn, the Gaang also learned to see some new perspectives. Slowly, all their destinies began to change.

None of them expected so many truths to come through the lies.

A For Want of a Nail AU. The story can be found here.

Tropes:

 * Absurdly Sharp Blade: Played straight at first, then subverted in Chapter 14, when Zuko explains how he cut through the chains holding Aang.
 * Accidental Truth: See the story summary.
 * Annoying Arrows: Subverted. The reason Zuko isn't bothered by/doesn’t notice the arrow initially is due to nerve damage. Also somewhat averted, since once he knows about the arrow, he knows that he needs immediate medical attention.
 * Badass Normal: Zuko, in his persona as Blue.
 * Badass Abnormal: Zuko!Blue when he is being assisted by.
 * Becoming the Mask: The more time Zuko spends as Blue, the more his Prince façade, his Blue façade and his 'true' self all start to mix together.
 * Believing Their Own Lies: Chapters 17 and 18, where Zuko confuses the hell out of Toph, and also ties himself in knots when Aang and Toph point out logical fallacies in his defense of his banishment.
 * Bonding Over Missing Parents: Happens between Zuko and Sokka and Katara early in the story.
 * Cast From Hit Points: What Zuko does to disable . A relatively mild example, however, as he recovers afterwards.
 * Change the Uncomfortable Subject
 * Chekhov's Gun: The medicine that Zuko gives the Gaang in Chapter 1 is used to treat Iroh’s wound in Chapter 17. Sewing is used to identify Blue after a long absence and a marked change in appearance, and Word of God is that sewing will pop up again.
 * Children Are Innocent: How Zuko views the Gaang after having gotten to know them better.
 * Cliff Hanger: Used infrequently but effectively.
 * Conflicting Loyalty: Zuko is slowly developing a case of this as he spends more time with the Gaang and becomes closer to them.
 * Cool Mask
 * Cool Sword
 * Cryptic Conversation: Every conversation Zuko has with  is one of these. Iroh is also fond of them.
 * Deal with the Devil/Bargain with Heaven: Zuko’s deal with  in Chapter 20.
 * Did Not Think This Through: Zuko doesn’t know how he’s going to keep Aang captive after they get out of Pohuai Stronghold.
 * Foreshadowing: Word of God has stated that  occasionally uses in communication with Zuko is foreshadowing.
 * The Guards Must Be Crazy: Played straight, subverted, and lampshaded. Whenever dealing with a difficult Fire Nation Officer, Zuko notes that, despite the inconvenience it places to him, the Fire Nation really needs more men like that. He even makes a mental note to follow one guy's career should he get back in a place of power.
 * Gossip Evolution: The whole reason the Blue Spirit's reputation gets more fearsome every time it's mentioned.
 * Hand Signals: Zuko and Mai have a large vocabulary of them. Toph and Suki also use some with each other.
 * Heal It with Fire: Zuko and Iroh both use the straight version; . The consequences of burning self in the straight version are handwaved through a) Iroh has Just That Much Control, and b) ability to apply incredibly helpful healing salve afterwards.
 * Holding the Floor: Toph pulls this in Chapter 22 to give the approaching Blue a chance to end the standoff between the Gaang and.
 * Honor Before Reason: It’s Zuko – what did you expect?
 * It Was a Gift: Zuko has a number of these. (He started out the story with the Earth Kingdom dagger and Mai’s heartstone; now he also has slate and bedroll.)
 * La Résistance: Sokka thinks that Blue might be working for a resistance group. There is also the Order of the White Lotus and the conspiracy started by.
 * Memetic Badass: Zuko!Blue, in-universe.
 * Moral Event Horizon: Zuko's whole attitude toward Aang understandably changes after the Siege of the North and the massive slaughter that someone he trusted and let walk free has inflicted on his countrymen. They've patched it up a bit, but Aang and Blue are still very definitely not friends anymore.
 * The Mutiny: stage a minor one after the Siege of the North when they go back to try and rescue any survivors of Koizilla’s attack. Also, it’s mentioned once that Zuko faced a mutiny from his crew about a year ago.
 * They're also plotting a long-range resistance conspiracy thing, largely in Zuko's memory while they believe he's dead.
 * Non Sequitur:  initial means of communication with Zuko tended to be inserting one of these into Zuko’s train of thought.
 * Oblivious Guilt Slinging: The Gaang does this to Zuko rather frequently with their little shows of trust in Blue.
 * Only Sane Man: Sokka is this for Aang and Katara when they first meet Blue. Also, Zuko believes he is this when he’s traveling with the Gaang as Blue.
 * Reassigned to Antarctica: Every member, they get assigned to a dead end patrol.
 * Royals Who Actually Do Something: The Fire Nation royal family, which isn't that different from their canon versions.
 * Shown Their Work/Word of God: Much of the background research and information for the story, as well as some of the future developments, are discussed in the comments of this post on the author’s Livejournal account. Warning: Heavy spoilers abound in the discussion.
 * Snowball Lie: See the story summary.
 * Invented Individual: Blue Spirit Lee.
 * The Speechless: Zuko!Blue is this in-universe.
 * Spirit Advisor:
 * The Stoic: The Gaang assumes Blue is this. He kinda is.
 * Supernatural Aid:  has given this to Zuko a few times.
 * This Means War: Invoked in Iroh’s thoughts when he finally learns that Azulon had ordered Ozai to kill Zuko and that Ozai had been going to do it.
 * The Voiceless: What Zuko!Blue is for the reader.
 * Warrior Prince: Zuko. Iroh used to be one.
 * The White Prince: Zuko, although he’s well on the way to becoming a Wise Prince instead.
 * The White Prince: Zuko, although he’s well on the way to becoming a Wise Prince instead.