Nai

Nai is a younger girl who is a teleporter, and manifests her abilities in the form of shadows. She also has a sensing power. Her name literally means "nothing". She has been blinded with a blindfold since birth to make her less prone to emotions, and to become an easy tool for Black Phantom. After Nai's brainwashing is removed by Kaoru's Force of Absolution, it is revealed that her head contained an explosive to kill her should she eventually escapes from Black Phantom's control, which is eventually removed by the combined efforts of Hyōbu, Fujiko and Sakaki. Since then her eyesight has improved and she now watches over Yūri, concealing her appearance by assuming the form of a black kitten which keeps her company. (Source: Wikipedia)