ItaSasu

ItaSasu is a Slash ship between Itachi Uchiha and Sasuke Uchiha in the Naruto series.

Canon
Itachi and Sasuke are brothers and have always been close. Sasuke looked up to Itachi and felt pressured to follow in his footsteps. However, he also felt awkward because their father Fugaku seemed to pay more attention to Itachi. Itachi also cared deeply for Sasuke since he was a baby. When the Nine-Tailed Fox attacked the village, the rest of the Uchiha Clan was gone so Itachi took care of baby Sasuke and promised him that he would be okay.

Sasuke was close to Itachi as he was growing up. Itachi helped Sasuke practice and carried him afterwards. When the time came for Sasuke to enter the Ninja Academy, their father refused to attend due to work yet Itachi missed an Anbu meeting in order to be there for Sasuke despite the protests of his father. However, Itachi began to have issues with the clan and Sasuke picked up on this.

Itachi was eventually ordered to wipe out the Uchiha Clan in order to prevent a coup d'etat. However, he was unable to bring himself to kill Sasuke because he loved his brother and knew that he had no part in the plan to overthrow the Hokage. Itachi convinced the Third Hokage to allow him to live before placing himself in exile. Sasuke was traumatized by having his entire clan wiped out and vowed to kill Itachi in revenge.

Years later, Itachi came to Konoha in order to find Naruto. Sasuke immediately chased him as soon as he found out and challenged him to a battle. He did not stand a chance against him and Itachi used the Mangekyou Sharingan on him. When Sasuke woke up, he was jealous of Itachi pursuing Naruto and left the village in order to train with Orochimaru so that he could become strong enough to defeat him.

Over two years later, Sasuke finally arranged a battle with Itachi. Itachi eventually passed his Mangekyou Sharingan to him before being killed. Sasuke considered himself the winner of the battle because of this. Later, he learns the truth about Itachi wiping out the clan and decides to take revenge against Konoha for everything that happened.

During the Fourth Shinobi World War, Itachi was resurrected by Kabuto. They confronted each other on the battlefield and Itachi explained why he spared his life. He then told Sasuke that he would love him forever. Sasuke later learned the truth about Itachi and the Uchiha Clan from the past Hokage and decided to defend the village because that is what his brother would have wanted.

Fanon
ItaSasu is a rather popular ship in the Naruto fandom. It is also one of the most popular incest ships in the fandom. While some do have a problem with the fact that Sasuke and Itachi are brothers and have a bit of an age difference, it still has a decent amount of fans as well. ItaSasu rivals other Sasuke pairings such as SasuNaru.

Trivia

 * Itachi was the person whom Sasuke loved the most until he married Sakura and has Sarada.