Masaru Kato is a central figure in Gantz, recognized as a secondary protagonist in the first phase of the story and the primary protagonist in the second. He is a tall young man with black hair and a toned, athletic build. Kato comes from a difficult family background; he and his younger brother live with their aunt, who views them as burdens. This stressful and abusive home environment motivates Kato to save enough money to get his own apartment, where he can properly care for his sibling. The death of his parents in a car accident has instilled in him a fierce sense of responsibility for his little brother, making his survival in the Gantz missions a matter of desperate importance.

Personality-wise, Kato is defined by his profound gentleness and strong moral compass. He is known to his teammates as an understanding and kind individual, someone who will only resort to violence as an absolute last resort. This pacifistic nature is a core part of his identity, as he is constantly searching for an alternative way to complete Gantz's missions that does not involve simply killing the designated targets. His weapon of choice reflects this aversion to lethal force; he prefers using the Y-Gun, which captures and teleports aliens away rather than killing them. Despite his gentle disposition, Kato possesses incredible physical strength and is highly skilled with all of the Gantz equipment. When he sees other people in danger, his hesitation vanishes, and he can be persuaded to use lethal force to protect the innocent. His imposing height, combined with his quiet yet powerful personality, makes him a natural leader who has a visible and calming effect on other players.

Kato's journey in the story begins with his death. He and his childhood friend Kei Kurono are killed by a subway train while trying to rescue a drunken homeless man who had fallen onto the tracks. This act of selfless heroism is what brings them into the deadly game orchestrated by the black sphere, Gantz. From the very first mission, Kato tries to reason with the alien targets, attempting to apologize for the death of its child, which nearly costs him his life. This incident highlights the deep bond between him and Kurono, as Kurono steps in to save him. Kato strives to be like the version of Kurono he remembers from their past, a person of strong character before he became disillusioned.

His relationships are a driving force for his actions. The most significant is his friendship with Kei Kurono, which forms the emotional core of the series. Early on, Kato's moral example begins to reshape Kurono's initially selfish and nihilistic personality. He is also fiercely devoted to his younger brother, and this responsibility weighs heavily on him, making his fear of dying in a mission all the more acute. Other players, such as Kei Kishimoto, look up to him as a gentle giant.

Kato's development is marked by tragedy and revival. During the mission against the Buddhist Temple aliens, he is killed by the final boss, leaving Kurono as the sole survivor. He remains dead for a significant period until a teammate known as the Old Man uses his accumulated one hundred points to revive him. Upon his return, Kato's role in the narrative expands. He becomes the primary protagonist, now driven by an unshakable goal to revive his best friend, Kurono, who has since died. This mission fuels his determination through incredibly dangerous encounters, including a large-scale mission in Osaka where he temporarily takes charge of the Tokyo team. His resolve is tested when he is abandoned by his teammates who prioritize their own survival, but a near-miss with a family in danger reaffirms his commitment to protecting others and achieving his goal. Ultimately, Kato succeeds in defeating the immensely powerful one hundred-point alien and uses the reward to bring Kurono back to life.