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 was one of the first mutants discovered by Professor Xavier. She has been a member of the original X-Men teams, Excalibur, the Exiles, and a cross-dimensional X-Men team similar to the Exiles known as the X-Treme X-Men.

Years later, Sage meets up again with Xavier, who at the time is recruiting the original X-Men. Instead of being chosen for the first X-Men class, she is instead sent to spy on the Hellfire Club as Tessa, and to keep watch on Sebastian Shaw, something which returns to haunt her in the following years.  While working at the Hellfire Club she encounters Psylocke, Jean Grey, and Emma Frost, who at this point is the White Queen. While she is able to successfully warn Psylocke away from the Hellfire Club, Tessa is later remorseful that she is unable to similarly prevent the corruption of Phoenix into the Black Queen. This transformation is a key factor in the creation of Dark Phoenix.  Sometime during Tessa's time with the Hellfire Club, Sebastian Shaw and Emma Frost dare the man known as Elias Bogan to wager in a game of poker. Though he holds no official rank in the Inner Circle, the telepathic Bogan is one of the Club's most important and powerful members; his patronage guarantees success in the organization. By the terms of the wager, if Shaw lost, Frost would "belong" to Bogan, but if Bogan's agent lost, then his fortune would be turned over to Shaw. Thanks to Tessa's advice and analytical skills, Bogan loses the game and is forced to honor the wager. Having never lost before, Bogan harbors a grudge against Tessa.  During the X-Men's initial conflicts with the Hellfire Club, Tessa primarily maintains a background role and is seen as only Shaw's unusually intelligent assistant. Hints of her more complex role are indicated when she and Xavier are kidnapped by Donald Pierce, a renegade member of the club, with grudges against both organizations. Based on the interactions between Xavier and Tessa, it is unclear whether he is helping to maintain her cover or whether he feels as if he has lost his agent, since the interaction between the two is kept impersonal. The two are rescued by the nascent team of New Mutants, and Tessa takes Pierce into custody on behalf of the Hellfire Club.  Tessa's next notable interaction with Xavier's students happens months later when the X-Men and Hellfire Club are hunting down Rachel Summers after her attempted murder of Selene, the newly installed Black Queen. In the middle of their fight, both teams are taken by surprise by Nimrod, a mutant-hunting robot from the future. Recognizing the more immediate threat, the two groups join forces and are able to severely damage Nimrod. The X-Men and Hellfire Club each sustain heavy losses and are forced to flee the scene quickly. Tessa offers Storm and the X-Men temporary sanctuary with the Club, which later results in Magneto and Storm jointly accepting the role of White King in the Inner Circle.

So she created Dark Phoenix?
unable to similarly prevent the corruption