Sai is first seen as an orphan who is recruited in a secret facility where they train you to have no personality or personal connections. The facility realize his extraordinary drawing ability and they encouraged the talent. Thats why his attacks are based on using ink for drawing in battle.
He is sent to the Village Hidden in the Leaves to join Team Kakashi. He soon leaves to the nearby twolegplace and finds a new den. He is often seen drawing pictures.
One day he is attacked by Gaara and they have a brutal fight. He ends up winning by having biting Gaara and sending poisonous ink into his system, paralyzing the tom. He ends it by clawing his throat and Pein comes, telling him the battle is over and he won. Sai disappears.
He is assigned to watch Gaara closely by the ANBU, he fights Gaara again and wins again. After a long time, Gaara talked Sai out of the ANBU and he quit after meeting Robin.
Robin is shown to be breaking his shell of being emotionless. Sai starts developing feelings for her after they hunted for an owl and she was getting attacked by it, he had saved her and after it they talked after they had killed it and eaten it.
He finds out Sakura and Naruto were murdered and he is sad.
He goes to the river with Robin.