Ernst
It was hinted in Morrison's run that Cassandra Nova took on the form of Ernst once she was placed inside the body of Stuff as part of her reprogramming. The final hint was in Here Comes Tomorrow (New X-Men #153), where she tells Martha Johansson, who Ernst was always seen around, that "of course you can still call me Ernst."
Subsequent writers have tried to contradict this, under presumed editorial edict. In Chuck Austen's New X-Men, Cyclops and Beast investigate Cassandra's containment unit following the destruction of the mansion, only to never find it (and seeming not to care once they don't); this would seem to suggest she was either not Ernst or they didn't know she was Ernst (Xorn, Ernst's own teacher, did not know but suspected Ernst was more than she seemed). Joss Whedon's Astonishing X-Men apparently contradicts it by showing Cassandra Nova still in Stuff's form. Likewise, Ernst has been distinctly shown alongside the student body in other publications.
Read more about this topic: Cassandra Nova