According to his own admission, Guido was a nerdy-looking kid in his early life, bullied by other children. He discovered his mutant ability at a young age, when he was struck by a school bus, and no one bullied him again.
Guido's parents were frugal, saving all their money for the future and insulting their child because of his size and his spendthrift and hedonistic attitude. A falling satellite crushed them, however, Guido settled out of court with the owners of the satellite for a vast amount of money. For some time, he lived a life of luxury until his finances were wiped out by bad investments. To make ends meet, he took a job as a bodyguard for the intergalactic thief and rock star, Lila Cheney.
When Lila vanished into outer space for some time, Guido was minding her house when the Dazzler, a member of the X-Men, washed up on shore nearby, a victim of the mystical portal known as the Siege Perilous. Guido helped Dazzler regain her health and memories as well as her singing and movie career. However, Dazzler's career soon crashed due to the public’s growing anti-mutant sentiment, and both Guido and Dazzler were contacted by allies of the X-Men, since the team was at the time thought to be disbanded, its members dead. In reality, however, the X-Men's allies were being held in thrall by the Shadow King on Muir Island, and Guido became one of the Shadow King's unwitting slaves. The X-Men eventually returned to defeat the Shadow King and rescue his subjects, including Guido.
Guido was invited to join a new government-sponsored mutant team which took the name X-Factor. He dubbed himself "Strong Guy" during a press conference and soon became a media darling. As Strong Guy, Guido served in X-Factor for many months, until one battle against aliens who were after Lila Cheney. He threw his body on a bomb, absorbing the blast at the cost of overexerting his mutant powers. His body grew to such a point that he suffered a massive heart attack. Rendered comatose, Strong Guy spent several months in recuperation. Eventually, he recovered, but X-Factor at that time had dissolved its government ties, and Guido retired from full-time crime-fighting.
Recovering, Carosella returned to Cheney's employ for a time before becoming involved with the underground mutant fighting league the Arena. Liberated by Storm of the X-Men, Carosella subsequently joined the X-Factor Investigations agency in Mutant Town run by his former X-Factor teammate and best friend Jamie Madrox.
The ever-jovial Guido Carosella uses humor to cover up the constant pain he's suffered from the moment his mutant nature manifested and deformed his body. Despite his less-than-serious nature, Carosella is a formidable fighter, having held his own against the likes of the Blob and the Hulk.