The tragedy of Emma Frost that was forgotten and unnoticed over the years, even by herself: The Wither Abandonment
Emma Frost endured several tragedies that marked her: her childhood, the deaths of her students, the Hellions, the Genosha massacre, and others. In all of them, she was powerless to save the people she loved, but there was one instance where she was able to avert their tragic fate: the life of her student, Kevin Ford (Wither).
Wither was personally recruited by Emma, as co-headmistress of the institute, for her squad of students, the Hellions. This came after she convinced him to return to the Xavier Institute following an altercation with Dani Moonstar, his previous mentor. Their relationship was one of direct mentorship, and they shared an extended period as part of the X-Men's academic structure. The interaction of their powers was even explored when it was discovered that Emma's diamond form was immune to Wither's "touch of death." She was one of the few people who supported him through his mutation, loving him unconditionally; in a sense, she was the maternal figure Kevin never had.
After M-Day, following an altercation with Wallflower, he believed his mutation had disappeared. Kevin suffered an emotional breakdown, and coupled with his classmates' distrust, he ran away from the Institute, not wanting to hurt anyone else. Emma Frost knew about this, but decided to keep it a secret and call X-Factor to keep an eye on him; this is understandable given that it wasn't a safe place due to Stryker's attacks. But even after that, even after Wolfsbane notified her of Kevin's disappearance... Emma did nothing. The events in San Francisco and Utopia happened, and she seemed to have no memory of Kevin. Meanwhile, Wither was seduced to the dark side by Selene Gallio, not through power or ambition, but for existential validation.
After their reunion during the events of Necrosha, Kevin and Selene's inner circle died at the hands of X-Force, with the exception of Blink. No one, not even Emma, mourned or even mentioned their deaths afterward; there wasn't even a reunion between them on Krakoa. The closest thing was her return in a Gwenpool comic, and that's how disappointing it was (at least for me).
The most unfair thing is that Emma could have actually saved Kevin, just like she did with Blink after Necrosha. Personally, I think it should have been Kevin, because Emma's relationship with Blink was more fleeting and specific. Their bond never had the depth or continuity that Emma developed with Kevin.