Sai

is a young ninja from the Foundation who specializes in techniques that use ink. He replaces Sasuke in Team Kakashi.

Background
Sai was orphaned as a child and recruited into The Foundation, a secret branch of the ANBU. There he was raised and conditioned to have no personality, personal connections, or a name, "Sai" being given to him for the purposes of joining Team Kakashi. Though largely successful in this regard, Sai retained an interest in art. When Shin, an older Foundation member, discovered Sai's drawing ability, he encouraged Sai's talent and they became very close.

Over time, Sai came to think of Shin as an older brother and looked up to him. Eventually, his "brother" died of an unnamed illness, and Sai's Foundation training suppressed most of the memories he had for his brother. Before Shin's death, Sai was making a picture book that would depict all the opponents that they faced and illustrate the feelings that they shared for each other. Sai's and Shin's portraits are located at opposite ends of the book, and in the center pages would be a picture of both him and his brother holding hands. He had forgotten that this was his intention, but after bonding with Naruto Uzumaki he remembers and is able to complete the book, causing Sai to give the first genuine display of emotion in a long time: a smile.

Personality
When first introduced, Sai was distinguished by total lack of emotion which he often masks with a false, cheerful, yet transparent facade. After much mental conditioning and training while in Foundation, Sai had suppressed almost all of his emotions, laying all of his loyalty to whoever commands him, and thus making it easier for him to accomplish dangerous, unpleasant, or unethical missions. Due to this training, both his own lack of social experience and personal initiative cause for Sai to have great trouble in understanding how bonds and friendships between people can motivate others. He often showcases this confusion after thinking about why Naruto would go so far to save Sasuke, even though Naruto had not been ordered to do so.

After spending more time with Team Kakashi, Sai comes to appreciate bonds, particularly the bond that exists between Naruto Uzumaki and Sasuke Uchiha. This causes him to break out of his emotionless shell and he starts to remember the times he had with his own "brother." In doing so, Sai was able to start giving genuine smiles. Wanting to experience personal bonds for himself, Sai sets out to make as many friends as he can. However, since he is learning most of this out of books and by trial and error, he often accidentally ends up offending people with his nicknames and conversation. He has come to develop his own, more pleasant, and thoughtful personality, while showing signs of more emotion, such as enthusiasm and occasionally nervousness. After his first time with Team Kakashi, Sai is seen being friendly towards others, helping companions when they need it, referring to peers kindly, and having fun in social situations.

Sai is very observant, and is always focused on his missions. He pays much attention to detail, allowing him to notice if something is not as it appears or should be, as shown from how he easily sees through Yamato's tricks.

Abilities
Sai's most notable talent is his skill as an artist, and he has claimed to have drawn thousands of pictures. Despite the great quantity and magnitude of the things he has drawn, he has never given one of his works a name, his lack of emotion making it impossible for him to form a significant enough attachment to his art to give titles. Even so, Sai's drawing ability is also the source of his jutsu, known as Super Beasts Imitation Picture. By infusing his chakra into the ink beforehand and drawing objects with a brush on his scroll (something he can do very quickly), he is able to bring his creations to life by animating the ink directly off the page. These creations grow to life-size once they have been brought to life--similar to Deidara's Exploding Clay constructs.

Sai typically creates giant birds for long range transport, and lion-like monsters for attack, or objects like umbrellas for everyday use. He can also make copies of himself for reconnaissance and transport himself to other locations by surrounding himself in ink. For discreet communiques when he is undercover, Sai is able to turn the words he writes into small animals, allowing them to travel to their intended target unnoticed. Sai is otherwise skilled in close-quarters combat.

Sasuke and Sai arc
Sai was assigned to join Team Kakashi by his superior Danzo to assist in meeting with a spy in Orochimaru's ranks to find out about Sasuke Uchiha. Sai is confrontational with his new teammates, questioning Naruto's manhood and insulting Sasuke. The latter causes Sakura Haruno to punch him in anger. As they continue on to the meeting point, Sai questions why Naruto is loyal to a traitor like Sasuke, to which Naruto said he's not a traitor, but a friend.

Upon reaching the designated meeting place, Yamato went ahead disguised as Sasori so that he could meet the spy. When the spy, Kabuto Yakushi, arrived with Orochimaru to kill Sasori, Sai, Naruto, and Sakura were called in for help. When Naruto and Orochimaru begin fighting, Sai leaves the rest of the team to begin his real mission. He observed Naruto's fight with Orochimaru, unconcerned with the danger he left Yamato and Sakura in, and approaches Orochimaru after he had repelled Naruto with a proposition from Danzo. Orochimaru accepted, and Sai accompanied him and Kabuto back to their hideout.

Once arriving at Orochimaru's hideout, Sai was introduced to Sasuke, who had been training under Orochimaru for the past two-and-a-half years. When Sai mentioned that he might get along better with Sasuke than he did with Naruto, Sasuke fixed him with a stare, startling Sai due to the amount of fear it gave to his emotionless self. He was led to his own room afterwards by Kabuto, who locked Sai in upon leaving. Soon afterwards, Team Kakashi tracked Sai down and confronted him about his real mission. Caught, Sai explained that he had been assigned to ally with Orochimaru on Danzo's behalf so that the two may join forces in destroying Konohagakure.

Disgusted by the ramifications of this plan, Team Kakashi retrained Sai before setting out to find and retrieve Sasuke. Sai, however, doubted their success, stating that Sasuke no longer cared about them. Naruto, in response, stated that he used to hate Sasuke, but over time he had formed a bond with Sasuke through their shared experiences, and as such he would do whatever it took to bring Sasuke home. Naruto's dedication to retrieving Sasuke struck a chord with Sai, who was left without words as a result. When Kabuto arrived and freed Sai, Sai instead helped to capture Kabuto, hoping to accompany Naruto to find out more about bonds.

Naruto and Sai split away from Sakura and Yamato to go looking for Sasuke. They are soon found by Orochimaru, and Naruto distracts him so that Sai can have a chance to scout on ahead. When Orochimaru retreats and Naruto reunites with Sakura and Yamato, Yamato goes through the belongings that Sai left behind. In doing so he discovers Sai's actual mission: to kill Sasuke as a traitor to Konoha. Team Kakashi races through the base to find and stop Sai, but Sai has already found Sasuke. Sai, however, truly has had a change of heart, and tries to capture Sasuke in order to protect his bond with Naruto.

Irritated by Sai's reasons for bothering him, Sasuke attacks Sai, drawing the rest of the team to their location. When they find out what Sai was trying to do, they all team up to restrain Sasuke. Although Sai is able to land a successful attack, Sasuke's abilities are too much for them and he prepares to kill them all. He is stopped by Orochimaru and Kabuto, who convince him to spare the team before leaving. Defeated, Team Kakashi returns home. Sai requests that he be allowed to stay a part of the team and, with Danzo's approval, begins spending time with his new friends.

Fire Temple arc
In the anime, Team Kakashi is sent on a mission to the Fire Temple. There they learn about one of the Twelve Ninja Guardian corpses being stolen, and are assigned to retrieve three more stolen Ninja Guardian corpses. They follow a trail to a grassy field, only to be ensnared in the enemies' trap, causing the entire terrain to transform into a rocky canyon. After being separated from his team, one of the tomb robbers, Fuen, plants a tracker on Sai and begins targeting him with several traps. Eventually, Sai discovers the tracker and places it on an Ink Clone to deceive Fuen. While failing to defeat Fuen, he managed to shake her long enough to go in search of his teammates.

He later finds Sakura in time to save her from being attacked by a giant spider. Sai's arm is broken in the process, but Sakura is able to heal the major damage. He also showed emotion (most likely for comic relief) when Sakura gave him the antidote for the spider's poison by jabbing a needle into his arm. He was was forced to take a temporary leave until his arm fully healed, and in the meantime was replaced by Sora. Despite his injured status, Sai still proved to be of use; when Asuma was having difficulty battling against Kazuma, Sai snuck up on Kazuma with one of his ink snakes and restrained him. Once he gets better and Sora leaves the team, Sai asks if he can be Sora's friend before he departs.

Hidan and Kakuzu arc
Sai begins visiting the Konoha library to read about relationships in the hopes of making some for himself. In the anime, this begins before and continues throughout the Fire Temple arc. When he reads that nicknames are a good way to make friends, Sai makes the mistake of uses nicknames that describe how he actually sees a person. When calling Sakura a "hag" earns him a beating, he concludes that women like to be called the opposite of what is true. To that end, he calls Ino Yamanaka "beautiful", something that receives another beating from Sakura.

Sai and the rest of the team are later sent to provide backup for Team 10 during their battle with Akatsuki. Because Shikamaru Nara was separated from the rest of the group, Sai and Sakura were sent as backup for him, though upon arrival they found that Shikamaru had already defeated his opponent. Sai returned with everyone else to Konoha, having not taken part in the battle. Because Naruto was injured during the battle he has difficulty eating when they get back. Sai, to improve their friendship, feeds Naruto, angering Naruto because he had wanted Sakura to feed him. Neither has their way when Kakashi arrives to feed Naruto instead.

Three-Tails Arrival arc
In the anime, Team Kakashi is once again sent to provide backup, this time for Team 8. Because the mission might lead to finding Sasuke, Naruto is not to be told about or take part in the mission. When he meets with Naruto he tries to use what he learned from a book to successfully lie, but to his own shame reveals the truth about the mission. Once they set out, Sai is tasked with finding Team 8. When they arrive at the location, they assist Team 8 in fighting Guren. During the fight, Sai distracts Guren while Naruto rescues Hinata.

The two teams then begin pursuing Guren's forces, only to become distracted by a huge tidal wave. Upon recovering, Sai and and Hinata discover that it is the Three-Tailed Beast. After receiving orders to seal the beast, Sai is assigned to prevent anyone from interfering. When he initially proves unsuccessful in this regard, he uses his ink creations to scout a wide area so that he can have proper warning of a future attack. Though he is soon incapacitated, he is able to alert Kakashi to an impending attack. Once Guren's team is finally dealt with and Sai recovers, he once again helps protect the sealing team.

Hunt for Uchiha
Sai has a diminished role in later arcs. As member of the eight-man squad assigned for the search for Itachi and Sasuke, he just remains as a tracking member team rather than employing any direct action to the mission. He is part of the 8 shinobi that confront Tobi, the weird and cunny Akatsuki member that tried to prevent all of them to intervene in the Uchiha Brother's fight.

Invasion of Pain arc
During the invasion of Pain, he is away with Yamato and Anko, tracking down Kabuto Yakushi. He seemed to use a special tracking jutsu to not be noticed by Kabuto. Anko even congratulated him for that. They stopped the search just after a surprised Yamato tells the group that Naruto has reached the 6-tailed form of the demon fox.

Sasuke and the Sixth Hokage arc
After learning that his master was appointed to become the Sixth Hokage, Sai immediately went to Danzo for new instructions. After being ordered to watch Naruto carefully, Sai hesitantly asked Danzo his plans for Naruto, worrying about his friend's well-being. This did not go unnoticed by Danzo. Later, Sai was confronted by Naruto and Sakura.

The two asked Sai to reveal all he knew about Danzo. Unfortunately, Sai revealed he was unable to talk about such things because of the Cursed Seal Danzo placed on all members of the Foundation. If he ever talked about Danzō or the Foundation, he would be permanantly paralyzed and unable to talk. He did however reveal to be surprised and dissatisfied with Danzo's decision to kill Sasuke. Quickly afterwards, Sai and his friends were attacked by two of the three Kumogakure ninja sent by the Raikage. They ordered for information on Sasuke, who they revealed to now be a member of Akatsuki and expressed their desire to kill the latter. This greatly shocked Sai and his friends.

Shippūden The Movie 2: Bonds
In Naruto Shippūden the Movie 2: Bonds, Sai, due to his ability to create flying mounts for himself with his Super Beasts Imitation Picture Technique, served a double function as a scout and first counterstrike against the Sky-nin attackers. He showed himself to have great proficiency in aerial combat, taking down the admittedly sparse forces the recuperating enemy sent out against him with relative ease. He even noted that he was getting bored from the enemies' poor performance.

Trivia

 * It was commented on by Sakura, Ino, and Naruto that Sai resembled Sasuke physically. However, Sai and Sasuke had totally different personalities and Sai has extremely pale skin and short hair compared to Sasuke.
 * As Sai was formally a member of ANBU, he referred to Kakashi as his Senpai out of respect.

Quotes

 * (To Sakura) "I've drawn hundreds, maybe thousands of pictures so far...other pictures have the situation at the time, and the artist's feelings...like portraits have the person's name...but don't pictures usually have titles...?...To be more precise, even if I try to give them titles, I just can't do it. I don't feel anything....they don't put me in mind of anything."
 * (To Naruto) "Please stop staring at me. Or I'll have to hit you."
 * (To Kabuto) "If people change...then so can I. But some things never change. Bonds. I'd like to learn more about them."
 * (To Naruto, Sakura and Tenzo) "Sai is the name I was given for this mission...I'm nobody. I am a tool for Danzō-sama. I don't exist."
 * "…I don't have feelings."
 * (To Sakura) "A smile can get you out of a tough situation, even if it is fake."