Piotr Rasputin was smuggled into the U.S. from Russia at age nine, for his own personal safety as opposed to anything threatening his family, most of whom still live in Russia. After a childhood described as violent and miserable, Rasputin discovered his mutant powers and began earning money by performing feats of super-strength for tourists. He also realized that he was gay. Eventually returning to Russia, he faced death from a Siberian firing squad, but was rescued and recruited by the Red Mafiya (the Russian Mafia). Not a criminal by nature but unable to resist the attractive profits, Piotr became an arms dealer and sent almost all his earnings to his family. When an arms merchant employed by Magneto's Brotherhood of Mutants double-crossed him during a transaction, Piotr exposed himself as a mutant by assuming metallic form to repel his attacker's bullets. In the aftermath of the battle, Jean Grey, who was tracking down potential X-Men, came upon the scene and recruited Piotr. As Colossus, Piotr proved an exceptional combatant against X-Men foes such as the Sentinels and Magneto's Brotherhood of Mutants, and he developed a crush on fellow recruit Wolverine.

After a few months of action Colossus decided the X-Men only valued his strength, and grew jealous of the romances among his teammates. He returned to Russia, vowing never to use his powers again; however, after Cyclops and Marvel Girl followed him and urged him to return, he rescued an endangered submarine, winning worldwide recognition. Colossus rejoined the X-Men, frequently teaming with Wolverine. When the Brotherhood of Mutants resurfaced, again led by Magneto, Colossus almost confessed his feelings to Wolverine during battle. When Magneto used his control over Wolverine's metallic skeleton to torment him, Colossus beat the villain savagely.

Although Colossus remains protective of Wolverine, he realizes his affections are unrequited. Piotr was briefly interested in Longshot, a mutant whom he and his teammates rescued in Krakoa, but later learned Longshot was a murderer. More recently, there have been hints of a romantic spark between Colossus and mutant speedster Northstar (Jean-Paul Beaubier); but as more of Piotr's friends pursue inter-team relationships, Colossus's self-doubt and feelings of isolation may yet return.