Brian Cruz was a young student at the Xavier Institute for Higher Learning, which is also the headquarters of the famed mutant team the X-Men. Tag was a member of the Hellions squad, which trained directly under Emma Frost, the school's headmistress. Tag was seen in various X-Men series, including New Mutants (volume 2, which focused on another squad) and the successor to this series, New X-Men. Tag was featured prominently in a own four-issue Hellions limited series.

Tag was de-powered as a result of the Scarlet Witch's manipulation of reality during the "House of M" storyline in 2005. As a result of losing his powers, Tag to be sent home. As the bus full of de-powered students was leaving the Xavier Institute, it was bombed by a follower of Reverend William Stryker, a Protestant religious leader with a deep hatred of mutants. Everyone on the bus, including Tag, was killed.

Nightcrawler, a devout Catholic member of the X-Men who had at one time been a priest, delivered the eulogy at Tag's funeral, which was held on the school grounds. A cross can be seen on Tag's casket, distinguishing it from most of the other caskets for the slain students.