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.