Mao is a Geass user who held a contract with C.C. before Lelouch, he has the power to read others minds but can't control it. A power that went out of control and drove him insane.
Little is known about Mao's past, but apparently as a little kid he was given the power to read the minds of others. However, he couldn't quite control this power and thus he could hear the thoughts of everyone around him, which nearly drove him insane. In order to cope with this, he walks around with a pair of headphones that constantly play the voice of C.C., but that has simply skewed his insanity towards a "yandere" quality where C.C. is involved. When C.C. does meet with him in the show, Mao plans to cut off her limbs with a chainsaw so she'll be with him forever, but Lelouch stops him.
However, he decides to get revenge by holding Nunnally hostage and threatening to blow up part of the school unless Lelouch can beat him in chess. He easily beats Lelouch in a game of chess, being able to read his mind, but Suzaku is able to save Nunnally and disarm the bomb while C.C. puts a bullet in Mao's head.
In the Nightmare of Nunnally manga, Mao is re-cast as a little girl with a synthetic Geass power called "The Refrain." This power allows her to essentially copy the effects of the drug from the main story, trapping the victims within a "prison of happiness" until they become brain-dead, but she ends up withering away from the side-effects of her own power.
Ichiro Okouchi was credited for writing Code Geass although original characters came from designs by Clamp.