Rozin

Rozin is a Slash ship between Roku and Sozin in the Avatar: The Last Airbender series.

Canon
Roku and Sozin were childhood friends. They were born on the same day and always celebrated their birthday parties together. Roku and Sozin would often practice their firebending together and Sozin usually won. When Roku was nervous around Ta Min, Sozin tried to encourage him to talk to her. Later, at their sixteenth birthday party, the sages arrived and announced Roku as the next Avatar. When he was feeling down and sitting in his room, Sozin visited him and tried to cheer him up. He ended up giving Roku the crowned prince headpiece before Roku would leave for over twelve years to master the other elements. He wore the headpiece that Sozin gave him every day.

Roku returned to the Fire Nation after being gone for twelve years. He soon visited Sozin who had since became Fire Lord and they still considered each other to be friends even after all of those years. Sozin later attended the wedding between Roku and Ta Min and tried to talk Roku into spreading the prosperity of the Fire Nation to the rest of the world. Roku was against this idea and warned Sozin to never bring it up again. However, he later discovered that Sozin had been colonizing areas in the Earth Kingdom. Roku walked into the throne room to confront him on this but Sozin was angry at Roku for the way that he addressed him. The two battled and Roku won but spared Sozin's life.

Many years later, the volcano on the island where Roku was living began to erupt. Roku attempted to stop the lava flow and save the homes on the island while everybody else evacuated. Sozin saw the eruption from a distance and arrived to help his friend. However, the volcanic fumes got to Roku and he fell over while begging Sozin to help him. Sozin realized that he could finally colonize the Earth Kingdom with Roku out of the way and left him to die. However, when Sozin was on his death bed, he ended up regretting what he did to his dear friend.

Fanon
Rozin is a rather popular ship in the Avatar fandom. It is one of the more popular slash ships and is popular among fans who live slash ships. Many fans prefer it over Ra Min because not as much is known about Roku's relationship with Ta Min and the episode about Roku's past focused on his relationship with Sozin. Some feel that there may have been more between them than meets the eye. Rozin most commonly rivals the Ra Min ship.

Trivia

 * Roku and Sozin's families would eventually join together after Ozai and Ursa married and had two children.
 * Sozin supposedly regretted betraying Roku when he was on his death bed.