When first introduced, Sai was distinguished by his total lack of emotion which he often masks with a false, cheerful, yet transparent façade. After much mental conditioning and training while in Root, 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 and his lack of social experience and personal initiative, Sai has great trouble in understanding how bonds between people can motivate others. He often showcases this confusion after thinking about why Naruto would go so far to save Sasuke, a traitor, even though Naruto had not been ordered to do so.
Sai's social awkwardness tended to get people angry, because he would often speak without considering his words and their effects on people. This was due to his inability to realise the difference between a rude statement and a true statement. Sai had read somewhere in a book that giving somebody a nickname would bring you closer to that person and develop your friendship, this resulted in Sai hurting Sakura Haruno's feelings, because he called her ugly, stating the first true character trait that came to his mind. This also nearly led to enraging Chōji, while eating barbecue with Team Asuma and Team Kakashi he nearly called him "fatso". Luckily Naruto covered his mouth before he could finish.
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", Shin. 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 a more genuine, pleasant and thoughtful personality, while also 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. The greatest change in his personality, however, came after he encounters his reincarnated brother. When Shin is used as a living bomb by Deidara, Sai becomes enraged and attacks mercilessly, showing genuine rage on his face. Sai also displays genuine sadness, shedding tears when his brother is freed from the technique and disappears.
Sai is very observant, and is always focused on his missions. He pays a lot of attention to detail, allowing him to quickly notice if something is not as it appears, as shown from how he easily sees through Yamato's tricks. This awareness spans beyond the trials of duty, as Sai was later quick to understand the nature of Naruto's inner burden. What he failed to understand, however, was that Naruto was not trying to save Sasuke just for Sakura. Naruto simply wanted to save his friend. Sai holds Naruto in high regard, and tries to assist Naruto whenever he's going through hardships.
Sai has short jet-black hair, ink black eyes, and very pale skin. He usually is seen carrying a small backpack with his brush, scrolls, and ninja ink. He wears a short black jacket similar to Fū and his partner Torune, most likely showing his allegiance to Root. He also carries a tip-less tantō on his back. The rest of his outfit consisted of a high collared midriff shirt, black pants, shinobi sandals and gloves with his index and thumb fingers exposed most likely to facilitate the use of his drawing based techniques. He had Danzō'scursed seal on the back of his tongue until Danzō's death, which caused it to disappear. Naruto, Sakura, and Ino said he physically resembled Sasuke, despite his shorter hair and paler skin-tone. When he's not on missions, Sai wears a casual outfit consisting of a black shirt with a high collar, matching black pants, and his regular shinobi sandals. He doesn't wear his forehead protector or gloves with this outfit.
As a member of Root, Sai is a highly capable ninja, considered by Danzō himself to be the strongest member of his generation. Danzō even had enough faith in Sai's abilities to task him with single-handedly killing Sasuke Uchiha. As an ANBU, Sai also uses a tantō.
To be able to execute his Super Beast Imitation Drawing technique, Sai carries around a specialised custom scroll to draw on, the scroll's roller has compartments for Sai's ink brush and to hold a supply of ink. A distinct advantage of this technique is that he can perform it with a single hand, leaving the other free for enemy counter-attacks. In the anime, Sai gets his ink from a supply store, which he says is the ink that works best for his Super Beast Imitation Drawing techniques. By infusing his chakra into the ink beforehand and drawing objects with a brush on his scroll, which he can do very quickly, Sai 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 Explosive Clay constructs.Sai's most notable talent is his skill as an artist, and he claims to have drawn thousands of pictures. Despite the great quantity and magnitude of the things he has drawn, he has never given any 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 talent is also the source of his technique, known as Super Beast Imitating Drawing.
According to Kakashi, Sai is one of Danzō's prized soldiers, and his Root training also included fūinjutsutraining. His sealing technique allows him to trap people within his paintings by drawing a tiger on a large scroll with a giant paint brush. Since it takes time to draw the tiger he needs allies to protect him while in battle.Sai typically creates giant birds for long range transport for himself or others,and lion-like monsters or giants for attack, or objects like umbrellas for everyday use. He can create snakes and use them as strong bindings on enemies, or mice to quickly scout an area. Sai can make clones of himself for reconnaissance. 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. He has also demonstrated the ability to create large scale drawings as shown during the battle of the two ambush squads; he created two large human beings which resembled the Benevolent Kings (仁王,Niō). These had enough physical strength to completely destroy Deidara's clay bird as well as send him along with Sasori plummeting to the ground. Sai can create explosive tags with his ink technique. He can draw long lines of tags attached to small birds, allowing him to bombard enemies from the air.
Despite not being his specialty, Sai has shown to be proficient in taijutsu. He was able to react and block Naruto's attempt to stab him with a kunai very easily despite having his scroll on his lap, and managed to capture Yamato during training. He also was able to subdue Kabuto Yakushi very easily and block Sasuke Uchiha's attempt to stab Naruto, Sasuke even commenting that he blocked the attack in the most appropriate manner. During Fourth Shinobi World War, he was able to break Shin's neck with a flying knee kick in mid-air. He is also very proficient with the Body Flicker Technique, and is capable of surprising even highly-skilled shinobi with it.