The character known as the Green Onion Kid is the first alien target encountered in the Gantz game. This being is a young, child-like member of the Onion Alien race, a species that has taken refuge on Earth while living in hiding among the human population. Physically, the Green Onion Kid has light green skin and dark green hair that sprouts from pointed, spike-like protrusions on his skull. He has red lips, black eyes, and appears to be a preteen or young boy. He is typically seen wearing casual clothes, including an orange shirt, brown pants, and white bathroom slippers, and he often carries a green onion with him.

In terms of personality and behavior, the Green Onion Kid is initially presented as friendly, timid, and non-aggressive. He seems to enjoy green onions and appears interested in sharing them or making friends, rather than posing any genuine threat to humans. He possesses some understanding of the Japanese language, though his command of it may be quite limited. When confronted by the Gantz hunters, his demeanor shifts to fear and self-defense, and he is even seen begging for mercy before being killed.

Despite his harmless appearance, the Green Onion Kid has notable defensive abilities. He can extend razor-sharp claws from his hand, which he uses briefly to wound an attacker in self-defense. He also has the ability to produce a thick, green, mucus-like substance from his body, which he uses to evade capture. He demonstrates significant durability, surviving a fall from several stories with only minor injuries, a trait likely stemming from his species' dense bone structure, an adaptation to a high-gravity home world.

The Green Onion Kid's role in the story is brief but pivotal, serving as the initial target of the very first mission for several main characters, including Kei Kurono, Masaru Kato, and Kei Kishimoto. He is identified by the black sphere known as Gantz as the mission objective. His death at the hands of the Gantz hunters is a brutal and tragic affair, as he is shot repeatedly while attempting to flee and plead for his life. The killing of this defenseless child-like alien directly triggers the arrival of a far larger and more powerful adult Onion Alien, presumably his father, who becomes enraged upon discovering his son's remains. This adult alien then proceeds to massacre most of the Gantz team in retaliation. The Green Onion Kid's death serves as a critical early moment in the series, highlighting the morally ambiguous and brutally violent nature of the Gantz game, as the hunters are forced to kill what appears to be an intelligent, peaceful, and frightened young being. He has no significant relationships beyond his implied familial bond with the adult Onion Alien, and due to his very early death, he undergoes no character development within the narrative.