Sai, known as Phantom Thief X (怪盗X Kaitou Sai) is his name formed from the words Monster Robber X.I. (怪物強盗 X.I. Kaibutsu Goutou X.I.). This is abbreviated into the name 'Kaitou Sai', and thus X is pronounced Sai throughout both the Anime and Manga.
Sai is a mysterious human with extreme strength, speed and the ability to alter his own cellular structure; because of this, he is able to change his whole body into any human form available, and also grants him indestructibility. He is seen to survive multiple injuries which would easily kill a normal human, letting him survive from a fall of any height, stab wounds, or crushing. Sai is able to become an electronic being, which allowed him to absorb an electronic being called HAL which grants him HAL's Eye which allows him to control humans temporarily. He believes himself to be without identity and in a desperate attempt to find one for himself, starts killing people apart and studies their insides to find out what gives someone an identity. He puts the remains of human intestines in the red box. Sai is intrigued by Neuro because he is a demon who he believes will answer his problems. With that reason, he chases and tries to kill Neuro to see his insides.
In the anime,
[Spoiler] Sai was born from a woman named Seiren who was suspected of being a witch. As a baby, he witnessed his mother's death, becoming crushed between stone slabs into a box. This resulted in his obsession with putting killed people in boxes. In the end it is mentioned by Yako that he is her half brother. [/Spoiler]
In the manga,
[Spoiler] it is revealed that Sai is an altered clone of Sicks and was actually born as a girl. His mother was killed after she gave birth to him, and when he was released into the world, he was tracked by the New Bloodline. However, after meeting Ai, who organizes his life and helps him keep the Phantom Thief X personality, the New Bloodline loses track of him completely and Sai's free to roam the world as the famous Phantom Thief. In the end, Sicks manages to track down Sai and capture him again. Sicks then erases his memories of being the Phantom Thief and clones his cells and implants them into the bodies of various members of the New Bloodline. Sai is later revealed to have been cloned into his "original" form by Sicks, that of a teenage girl and has gained the ability to read memories. Sicks later renames him XI (eleven). Later, Sai regains his memories and finds his true form as the self he used to be as the Phantom Thief, thanks to Yako. He then shows kindness by transforming into the dead Sasazuka and gives Yako an opportunity to grieve for him. In the end, Sai dies and is buried by Godai and Yako. [/Spoiler]
