LeaIsa

AkuSai is the slash ship between Axel and Saïx in the Kingdom Hearts fandom.

Canon
Lea and Isa both lived in Radiant Garden and were best friends. They always got into trouble together. One day, they encountered a boy named Ventus. Lea befriended Ven while Isa thought that it was odd that Lea wanted to make so many friends. Lea comments that he was a lot of friends so that he is never forgotten while Isa admits that he could not forget Lea if he tried. The two then left towards Ansem's Castle. They were caught at some point and thrown out by Dilan. They also buy sea-salt ice cream and eat it together.

At some point, Lea and Isa are turned into heartless. Their strong will also caused nobodies looking exactly like them to be created. Xemnas recruited their nobodies and named them Saïx and Axel. They were made numbers VII and VIII of Organization XIII respectively. Axel and Saïx are friends as they retained the memories of Lea and Isa. They also made plans to eventually take over Organization XIII for themselves.

Roxas eventually joins Organization XIII and Axel befriends him. He slowly begins to acquire a heart because of this. Axel also uses his friendship with Saïx to manipulate him into doing things for them such as allowing Roxas and Xion to be on missions together. As Axel goes closer to Roxas, he begins to drift farther from Saïx. By the time that Roxas left the organization, Saïx gave up on his plans with Axel as he believed that it was just a fantasy and that Axel had changed too much.

Axel initially continued to be loyal to the organization but eventually left in order to help Sora whom Roxas had merged with by that point. Saïx later interrupts a conversation between Sora and Axel which forces the latter to flee. He assured Sora that Axel will be given the maximum punishment. Axel eventually sacrificed his life to help Sora get into the World that Never Was and he eventually destroyed Saïx.

Lea and Isa eventually appear whole again in Radiant Garden and wake up where they were turned into Heartless. Braig and Isa wake up first and flee the scene. Isa ends up taking the name Saïx and joins up with Master Xehanort. Lea then wakes up and immediately wonders where Isa is. Ienzo tells Lea that Isa can't be whole but Lea still insists on looking for him. Lea later saves Sora from becoming another vessel for Xehanort and Saïx eventually attacks him, revealing that he is willing to become a vessel for Xehanort. Lea is shocked and saddened by this.

Fanon
AkuSai is a very popular Kingdom Hearts slash ship but is not quite as popular as AkuRoku or XemSai. However, it is still very popular. This especially became the case when it was revealed that Axel and Saïx were once very good friends until Roxas came between them. Once Lea and Isa were introduces as best friends in Kingdom Hearts Birth by Sleep, support for this pairing increased further. Some fans hope that Isa can still be saved as they do not believe that he really wants to become a vessel for Xehanort.

Trivia

 * Axel and Saïx are thought to represent the sun and the moon, even though Axel's element is only fire and not explicitly the sun.