Ernst is a mutant student of the Xavier's Institute. No information could be found on Ernst's life before becoming a student nor could the nature of her genetic mutation be fully identified or understood.
Ernst was created by Grant Morrison and Frank Quitely and first appeared in New X-Men issue 135 (2003).
Writer Grant Morrison had intended Ernst to be Cassandra Nova, something he hinted at throughout his run but never ultimately resolved. Subsequent writer Joss Whedon instead made this no longer the case with Cassandra Nova instead residing within Emma Frost.
Mayor Story Arcs
Ernst, who appears to have befriended her disembodied fellow student Martha Johansson, briefly joined the Brotherhood founded by the drug-induced Magneto when he went on a destructive rampage in New York. She ultimately rebelled against Magneto and allied with the X-Men.
Ernst is one of the few mutants who have retained their powers after the events of M-Day. She continued living with the X-Men, though she didn't make the cut for the New X-Men team because she was excused by Emma Frost from the battle royale. She joined the X-men once more when the re-located to a former base of Magneto, named Utopia. This was after the original school was destroyed in the events of Messiah Complex.
Wolverine and the X-men
Recently Ernst joined Wolverine and many of the students back to the ground of the original Xavier's Institute, now known as the Jean Grey School.
Powers and Abilities
While she appears to be an elderly woman, Ernst is actually a young girl. Ernst may suffer from progeria, a genetic disorder that results in premature aging; however, it is also possible that her appearance is a result of her mutant nature.
Earth-15104: Here Comes Tomorrow
Martha Johansson was accompanied by Cassandra Nova, who she would often call "Ernst". Whether Ernst was Cassandra Nova or it was simply a result of Martha's Alzheimer's disease was never explicitly stated. The two perished in their final battle against Sublime and the truth was never revealed.
X-men: Battle of the Atom
Ernst is featured in the mobile card game "X-Men: Battle of the Atom" based on the comic book story with the same name. Her cards are: