The original five X-Men -- Cyclops, Marvel Girl, Beast, Angel, and Iceman -- have been fixtures in the franchise since their introduction. Their status as the first prominent students of Charles Xavier is well established -- but there's another young mutant who should have been among their number.
Over the years, retcons revealed that Sage was actually something of a precursor to the X-Men -- even saving Xavier's life at a crucial moment, ensuring the X-Men could even be founded. This is how Sage's history quietly makes her one of the most important recruits Xavier ever had and highlights how untrustworthy he can be.
Sage, aka Tessa, technically debuted in Uncanny X-Men #132 (by Chris Claremont and John Bryne). At first, Tessa was just an unnamed woman who was a part of the Hellfire Club -- becoming a more important figure as time wore on. She worked with the X-Men and became an ally to the New Mutants while they worked alongside the Hellfire Club for a bit before members of the group became more heroic. Eventually revealed to be a mutant herself with telepathic abilities and a computer-like mind, Sage became a voice of reason before being retconned into a surprisingly important figure in the hidden history of the X-Men.
X-Treme X-Men #44 (by Chris Claremont and Igor Kordey) explored her true origins -- revealing that roughly twenty years before the present day, a young Sage was in the Hindu Kush, struggling to survive in a war-torn region. Hearing a voice from a nearby cave, Sage found Charles Xavier -- paralyzed from his battle with the alien villain, Lucifer. After speaking to Xavier she eventually decided to help him, saving his life. The pair would be reunited years later when Xavier was in the process of forming the original X-Men. One of his potential recruits was Sage, who he quickly trained.
In fact, Sage was potentially poised to be a founding member of the X-Men -- and in many ways serves as a precursor even to characters like Cyclops, who is often considered the first X-Man. But thanks to the fast mastery of her powers and more pragmatic approach to survival, Xavier gave her a different role. He convinced Sage not to become an X-Man, but instead to serve as his spy in the Hellfire Club. She did her best to help the X-Men and their allies, but Xavier's decision to employ her as an agent instead of a student left her troubled, as she wrestled with her belief that he only saw her dark side. In the present, Sage has openly revealed her allegiances and currently works with X-Force -- with her morality driving a wedge between her and another founding X-Man, Beast.
Sage's place as an early X-Man candidate is an interesting addition to the origins of the X-Men, as it highlights its founder's secret machinations. While his intentions have largely been good over the years, he's been called out repeatedly for the secrets he's kept -- especially since that habit has continued even into the present day. It also makes Sage one of the most quietly vital mutants in the team's history. Despite her relatively small role in the overarching history of the team, her saving Xavier in the mountains makes her a crucial player in the formation and achievements of the X-Men. Without her, Xavier might not have ever escaped those mountains, and the X-Men wouldn't exist.