SaiSaku

SaiSaku is a Het ship between Sai and Sakura Haruno in the Naruto series.

Canon
Sakura and Sai first met when the latter was assigned to their team in order to replace Sasuke. Sakura was initially pissed off when he called her ugly. However, she later tried to be open-minded towards Sai but punched him when he insulted Sasuke. Sai eventually betrayed the team for his true mission and ignored Sakura when she was about to take a fatal fall. She retrieved his sketch book and attempted to gather clues about him.

Sai eventually comes around and decides to help Team Kakashi as he desired to learn more about human bonds and make some of his own. Sakura was sympathetic of this and invited him to meet Kakashi with her. He called her ugly again but was willing to shrug it off. Sakura eventually took Sai to meet Team 10 and was irritated when he called Ino beautiful. She was hoping that he would insult her rival in the same manner but he decided to try the opposite.

Sai and Sakura were later eating with the team when Konohamaru came along. After punching him for using another Sexy Jutsu, Konohamaru used a male variant that involved a naked Sai and Sasuke in a suggestive pose. This aroused Sakura and caused her to display a similar perverted reaction to Naruto. Sai just stood there in bewilderment was not surprised by this.

After Pain's attack on Konoha, Sai confronted Sakura over Naruto's promise to bring Sasuke back. He explained that Naruto loved her and that Sakura was causing him a lot of pain. Later, during the Fourth Sinobi World War, Sasuke decided to side with the village again. Sakura smiled and stated that she thought he was sincere but Sai could tell that she was hiding her pain and doubt.

Fanon
SaiSaku has a decent amount of popularity. It is not as popular as NaruSaku or SasuSaku but it still has a decent number of fans. SaiSaku is mainly popular due to the fact that Sakura was the first to noticed the fact that Sai looks like Sasuke whom she was in love with. There is also the fact that Sakura is one of the first people Sai got along with and the umbrella moment. SaiSaku rivals the SasuSaku, Narusaku and SaiIno pairings.

Trivia

 * Despite his small understanding of human emotions at the start, Sai is one of the only people who can see through Sakura's fake smiles.