The Onion-Alien is the name given by the Gantz system to an extraterrestrial species that serves as the very first target of the game in which recently deceased humans are forced to hunt aliens. Two members of the species appear in the story: a juvenile known as the Kid Onion Alien and a large adult who is strongly implied to be his father. The two live quietly in a rented apartment in a residential area of Tama City, blending into human society because their bodies are very similar to those of humans. Members of the species have pale white skin and green hair, giving them physical traits that resemble onions, and they survive almost entirely on green onions, leeks, and similar vegetation. This diet causes them to emit a powerful, unpleasant odor, and their bodily fluids are green and foul-smelling.

The juvenile Onion-Alien is timid and friendly by nature, although his appearance, with half-lidded eyes, a crinkled forehead, and frowning lips, tends to provoke and unsettle those around him. He constantly eats onions and, because young members of the species have difficulty regulating their internal fluids, he often leaks a rank substance from his nose and mouth. When threatened, he can spray this onion juice at attackers as a distraction, and he can extend short, sharp claws from his fingers. His recurring statement when he feels powerless is that onions are enough for him, and his habit of offering an onion to pursuers functions as a sign of surrender. The adult Onion-Alien is far more formidable. As the species matures, its skin hardens until it is as tough as steel, and the adult possesses roughly ten times the strength and five times the running speed of an ordinary human. He can crush a human head with his grip, tear through bodies with long razor-sharp claws on his hand, withstand gunfire and being struck by a car, and produce a scream powerful enough to shatter nearby glass.

The two Onion-Aliens pose no apparent threat to the humans around them and are simply attempting to live in hiding while feeding on the vegetables they love. They become victims of the game's first mission when the inexperienced hunters, misled by a veteran participant who claims the mission is a television stunt, track down the juvenile. Despite his attempts to surrender and escape, the child is cornered and killed in a one-sided attack. The adult then arrives to discover what has happened to his child and, overcome by grief, slaughters nearly all of the hunters in revenge, demonstrating a strong protective instinct and deep bond with the younger alien. He is eventually restrained by capture weapons fired by another survivor and killed, bringing the mission to an end.

Within the larger story, the Onion-Aliens serve to introduce the moral ambiguity of the Gantz game. They are presented not as aggressive monsters but as sympathetic beings caught in an unfair system, and the willingness of the hunters to kill the frightened child forces the question of whether the aliens are truly the enemy. This establishes a recurring theme in which many of the creatures selected as targets are intelligent beings trying to live peacefully. The species is clearly defined by its love of onions, its strong smell, its emphasis on friendship, and its two notable abilities: the juvenile's onion juice spray and the adult's devastating physical power, including his razor claws and steel-hardened body.