About the character
Katou Masaru is one of the main characters of GANTZ. He was killed along with Kurono Kei, his best friend from his childhood, when they tried to save a homeless man (or hobo) from being run over by the subway train. After they climbed down to the tracks and pushed the man up, a train came by and they had to run because of the lack of time to climb up. They thought it was going to stop at the station, but unfortunately for them, it was a non-stop express.
After being killed by the train, Katou and Kurono were transported (much to their surprise) to a strange room with many other people who happened to have just died. The even stranger black orb in the room then announces to them that they are now at the mercy of its whims. They now have to battle the enemies directed by GANTZ, the black orb, or die (again).
Katou takes a totally different approach than Kurono on "the game." Instead of looking at it as something fun to do and a place where he can be powerful as Kurono does, Katou tried to keep everyone safe and get out of the game by reaching 100 points. He becomes a natural leader and in turn inspires Kurono to be come a good leader (after Katou's death).
Katou got his noble nature of protecting the weak initially from Kurono when they were kids. Kurono was strong and often fought with bullies, so Katou looked up to him. When Katou got in high school, he always stuck up for the small guys and often fought their bullies to protect them.
The people he is closest to are his little brother Ayumu and Kurono, whom he affectionly calls Kei-chan. The other person who cares most about Katou is Kishimoto Kei, a girl who died on the same day as Katou and Kurono, after she slit her wrists in the bathtub. After being transported to the GANTZ room naked, she was almost raped by one of the other guys there, and only Katou was brave enough to stand up and save her. Afterwards, she falls in love with him and even died to save him in one of their missions.
When under pressure to either do something and save someone, or be apathetic, Katou's trademark reaction is to sweat in the face and then to help the person in trouble. Of all the powerful weapons to pick from, Katou only uses the Y-Gun, a symbol of his peaceful nature, since the Y-Gun is the only weapon that does not kill/harm its targets--it only captures.
Katou is killed on one of their missions after he defeated all the aliens to save a dying Kurono, who ends up being the only one alive afterwards, and completely alone for the first time in his life. Katou is brought back later by someone from the game and a member of Kurono's team at Kurono's request using the 100 point reward.