Sage, also known as Tessa is a fictional character in the Marvel Comics universe. She has most often been associated with the X-Men and the Hellfire Club, whom she spied upon for Professor Charles Xavier.
A mutant, Sage possesses a number of mental abilities and was originally presented as the personal assistant to the Hellfire Club’s Sebastian Shaw, but an extended retcon revealed that she had been one of the first mutants Professor Xavier discovered (Sage says that this was about the same time as he located Beast, but in other accounts he found Sage first). Instead of being chosen for the first X-Men class she was sent to spy on the Hellfire Club, something that has haunted her in the years since.
She has been a member of the original X-Men teams, the X-Treme X-Men and more recently a member of the latest incarnation of Excalibur.