Neutron is a former member of the Imperial Guard, a multi-ethnic group of super-powered alien beings who act as enforcers of the laws of the Shi'ar Empire. Under his original Guardsman name of Quasar, he was amongst the first of the Imperial Guard encountered by the team of superhuman mutant adventurers known as the X-Men who sought to rescue the Princess-Majestrix Lilandra from her insane brother, then-Majestor D'Ken. Following the orders of their emperor, the Guard clashed with the X-Men and were on the verge of winning when the band of interstellar freebooters known as the Starjammers arrived to turn the tide of battle in the X-Men's favor.
Quasar was later amongst those Imperial Guard members who allied themselves with the treacherous Shi'ar High Council member Lord Samédàr who was aiding an attempted coup of the Shi'ar throne by Deathbird. Even after the ploy was discovered, Quasar, like most of the other Guard members present, still chose to side with Samédàr against the X-Men who were defending then-Empress Lilandra.
Later, after the formerly-exiled Deathbird had usurped the Shi'ar throne, Quasar was amongst those Imperial Guard members who clashed with the British team of costumed adventurers known as Excalibur and the Starjammers over the fate of the then-bearer of the cosmic Phoenix Force, the alternate future daughter of the X-Man Jean Grey named Rachel Summers.
During the war between the Shi'ar and Kree Empires, Quasar - under his new Guardsman name of Neutron - was part of a small team of Guardsmen who were charged with preventing the member of the Earth team of super-powered beings known as the Avengers named Quasar from retrieving the legendary Nega-Bands of the Kree warrior Captain Mar-vell, which had been stolen. Neutron has since had a long-standing grudge against the Avenger named Quasar, and once teamed up with the Russian super-being known as the Presence to seek his revenge.
Neutron was also amongst those Imperial Guard members who defended Lilandra against an assassination attempt by the Kree Ronan the Accuser and his unwilling agents, the royal family of the Earth race known as the Inhumans.
Following a meeting of the intergalactic council, Earth was declared a threat to the entire cosmos and consequently it was deemed to turn the planet into an intergalactic prison. Criminals from numerous alien races were teleported to Earth, among them the former Imperial Guard members who had allied themselves with Deathbird and Lord Samédàr all those years ago. These former Guardsmen had allied themselves with a lone member of the alien D'bari, calling himself Starhammer, who sought revenge on Jean Grey for the crimes of Dark Phoenix.
Recently, another member of Neutron's race, the Stygian named Cosmo, has been seen as a member of the Imperial Guard.