This character never reveals his name in order to avoid being arrested by the police. What kinds of crimes he would be arrested for is unknown, but he has said that he would be in prison for 20 years if he was caught again. He additionally spent some time in a Mexican prison (despite being innocent in that incident). His actual age is never confirmed, but he is implied to be in his thirties.
In his first appearance, he was the boss of the hijackers who hijacked the plane the Cromartie students were on. It was his group's first attempt at this crime, and they were thinking about turning themselves in when Yutaka Takenouchi made them go through with it so he could avoid being on a moving aeroplane. His plan to avoid flight failed miserabily, and the hijacker described on this page took Yutaka's place in school to avoid being arrested. Upon arriving at school, he drew the kanji seen on his mask - "bamboo" when written by itself, and the first symbol in the name Takenouchi, which may have been part of the reason why everyone but Akira Maeda was fooled by the disguise. After the other students realized that the hijacker was a fake (which took another trip, when both the Takenouchis were present), they let him stay around and started to call him "Mask".
During the series, he starts to distance himself more and more from his time as a criminal. He often gives the other students advice to avoid going down the path he took before, and tries to find non-violent ways to solve problems.
Although this only was touched upon on one short story, he has bad eyesight. It is likely that he changed to contact lenses after the other characters saw his glasses as a failed attempt at being funny.
He is performed by Takaya Kuroda (anime), George Manley (English dub) and Itsuji Itao (movie).