|Quotes||"Onions are Enough for me"|
Onion Aliens (レイカ, Negi-Seijin) are a race of aliens that have taken refuge on the earth and therefore have become targets of a global alien extermination effort.
The Onion Aliens were the first aliens Kurono, Kato, and Kishimoto, along with six other participants, had to face as part of their first mission in the Gantz game. The "Negi Seijin" (Negi meaning 'Green Onion' and Seijin meaning 'Alien') were true to their title, as they were green aliens who loved green onions. The first onion alien encountered was a small, child-like alien that was quickly disposed of even though it begged for mercy from the other participants, who proceeded to shoot at it with their X-Guns.
Upon killing the Onion alien child, its father, a larger and more dangerous version of the child alien, was outraged and massacred most of the players. Kato confronted the massive alien in an attempt to calm it down. This attempt didn't work as the alien could not understand Kato, leading it to strike Kato down. The alien quickly turned its sights to Kei and Kishimoto and began chasing them. Kei, using the power of his suit, became a diversion to the monster, quickly out running it and beating it down. As the Onion alien was restrained by the Y-Gun, Nishi was forced to send him "up" after Kurono refused to kill the alien himself.