Negasonic Teenage Warhead

Ellie Phimister was a teenage student in Emma Frost's telepathy class in Genosha.

During a tutoring session, Ellie informed Emma of nightmares she had 50 times in the same night where everyone in Genosha died. Instantly after she said this, Cassandra Nova's Sentinels wiped out all of Genosha. Emma, who survived the attack thanks to her secondary mutation, carried Ellie's dead body and said she will be a credit to her family and the mutant race. She then ordered members of the rescue team to bring the girl back to life, but they said she had been dead for hours.

In Astonishing X-Men, Ellie Phimister appeared to be alive and part of the Hellfire Club's new Inner Circle. However, it was revealed that the new Inner Circle was only projections from Emma's mind due to guilt from surviving the Genoshan genocide.

In X-Necrosha, the true Ellie was revived with an alien techno organic virus, only to be sacrificed to Selene.

This description - which I wrote myself - is taken from Marvel.com's Wiki.