Mao (Nightmare of Nunnally)

Mao (マオ) is a character that only appears in Nightmare of Nunnally. She has a Geass ability granted by an infusion of cell matter from C.C. A Geass sigil lights up in her left eye when she uses it. Her Geass ability is known as "The Refrain," (マオリフレイン, Maorifurein) which allows her to read and manipulate the minds of others through eye contact.

Personality
Prior to her betrayal of the Irregulars, Mao was sick of how the Irregulars treated her and her teammates. As a result of this, she betrayed the Irregulars and escaped to Saitama Ghetto where she offered her service to the people living in the Ghetto by using her Geass to let them relive past experience She described her acts as "helping" the people who were miserable but at the same time contradict herself by saying that pain is necessary in order to continue living and that she receives strength from it revealing a more complex personality behind her

Character History
Mao is mentioned by Alice that she used to be an Irregulars member just like her, before she left she took a large amount of Neutralizer with her. She first appears in chapter 11 in Nightmare of Nunnally trying to remove Nemo from Nunnally. Because C.C.'s cell matter causes cellular breakdown, she seeks to steal Nunnally's bond with Nemo, which she believes to be a possible cure. After Alice breaks Mao's remaining suppressants, before she died Mao curses Alice letting her know that this is the fate of all Artificial Geass User, then her body turn to dust and finally died.

Geass
Her Geass the Refrain in addition that it allow her to read the mind of other people and control it, it allows her to render her victim catatonic by forcing them to relive past experiences, similar to the drug in the anime it is named after. As an Artificial Geass User just like the rest of the Irregulars she also receive the same weakness as the rest of them, the C.C. Cell inside her will eventually reject her body causing the host to died from the process.

Knightmare
In one panel Mao seems to have a Sutherland as a Knightmare Frame. It is possible that the GX 01 series was still not introduced to the Irregulars around the time she left.