Sage was recruited by Professor X at the same time as the original X-Men but he trained her in secret as a spy rather than as part of the main team. He sent her to the Hellfire Club to keep watch on Sebastian Shaw. While working there, she encountered Psylocke and Jean Grey, as well as Emma Frost who was then the White Queen
Later, she joined the X-Men, acting as a living computer, able to remember everything she sees and hears and provide analysis. However, her main mutant power is the ability to 'jump-start' other mutants' abilities. She used this power to save Beast's life and give Rogue better control of her powers. She is part of the team searching for the Books of Truth.