ShikaSasu

ShikaSasu is a Slash ship between Sasuke Uchiha and Shikamaru Nara in the Naruto series.

Canon
Sasuke and Shikamaru were at the Ninja Academy together and graduated at the same time. Shikamaru immediately noticed that Sasuke was an Uchiha and saw a lot of potential in him because of this. Shikamaru secretly looked up to Sasuke but never told anybody about this. During the Chunin Exams, he was interested in watching his fight with Gaara. Shikamaru was later sent on a mission to stop Sasuke from chasing Gaara.

Sasuke later fled the village in order to go train with Orochimaru. Shikamaru, who had just been promoted to Chunin, was placed as the leader of a mission to retrieve him. After gathering his team, he explained that he never liked Sasuke but wanted to save him because he is a citizen of their village. However, he secretly thought about how he looked up to him and wondered why he would leave. The team ended up failing their mission and Sasuke made it to Orochimaru.

Over two years later, news got around that Sasuke allied with Akatsuki. Everybody in Shikamaru's age group discussed the possibility of killing him and he had to calm down Ino when she cried over him and felt responsible over his failure to stop him from leaving in the first place. During the Fourth Shinobi World War, Sasuke decided to defend the village but Shikamaru was unable to trust him. However, he decided that worrying about the war was more important.

Fanon
ShikaSasu only has a small amount of popularity in the Naruto fandom. It is not as popular as other ships such as SasuNaru or ShikaCho. ShikaSasu is mostly shipped because both are considered to be geniuses and some feel that they would look good together. There is also the fact that Shikamaru looked up to Sasuke and felt determined to stop him from joining Orochimaru.