Irko

Irko is the family ship between Iroh and Zuko from the Avatar: The Last Airbender fandom.

Canon
They are extremely close in canon, Iroh thinks of Zuko as his son and Zuko recognizes Iroh as being the one to be a true father to him. There are multiple instances where they save each other from getting hurt (ie. Iroh preventing Zhao from attacking Zuko, Zuko rescuing Iroh from the earthbenders, Iroh saving Zuko from Azula's lightning and etc.) Iroh has raised Zuko from when he was 13 to 16 and played a big part in what type of person Zuko turned out to be.

Fanon
Though it's well established that Iroh is the father Zuko never had and Zuko is the son Iroh lost there are some fans who ship them romantically. Most though just recognize their father - son bond. When Iroh was revealed to have ascended to the Spirit World permanently in The Legend of Korra, many fans wanted Zuko to reunite with his uncle in the Spirit World and some fanart has been drawn where the two reunite.

Fandom
FAN FICTION

Trivia

 * There isn't as much Iroh adopting Zuko fanfiction as you'd expect considering how close they are in the show and how most fans adore the relationship.
 * If you search up who Zuko's father, Iroh appears (along with Ozai) in the results although they are not "officially" father-son in canon.
 * Some fans like to theorize that Zuko's daughter, Izumi, calls Iroh grandfather.
 * Even before his son's death, Iroh has shown to be very close to Zuko.