I hate it when people call Zutara fans “delusional” when there was so much material released that hinted at the two being a canon ship and the couple was even considered by the production team on multiple occasions. I’m sure there’s plenty of other stuff in addition to this and I’d appreciate contributions. But y'know, haters gonna hate.
Sources:
1.http://katara-zuko.livejournal.com/1308178.html
2, 3, 4. From the official Nickelodeon Magazine
5. Sozin’s Comet: The Final Battle Novelization
6. http://sractheninja.tumblr.com/post/20865352542/inbetweenthelineart-kataangcaps
7. http://www.lastairbenderfans.com/index.php?start_from=&ucat=8&subaction=showfull&id=1269959997&archive=1272555212
8. http://korraisnottan.tumblr.com/post/80678500483/the-writers-never-considered-fucking-zutara
Another account of this event: http://colonel-mustang.tumblr.com/post/19385834309/little-riza-byron-the-dissectioneer-i-like
(From this class: http://www.decal.org/courses/1905)
9, 10. Avatar Extras (Book One and Three)
Bonuses (not to be considered as official)
Dante Basco confirms that Zuko was in love with Katara
Transcript: “Okay, well: I did fall in love with Katara. And I’ll probably always love her. But in the way you always hold a special place in your heart for your first love. And yeah, I love Mai. I really do. But if the world is really going to end, maybe we can get the old crew back together and save it again.”
https://soundcloud.com/rufiozuko/zukos-answer-to-the-end-of-the
Dante Basco says that Zuko and Katara are soulmates
http://karli21s.tumblr.com/post/66629314938/yo-theyre-soulmates-they-are-did-you-watch-the