When it was announced that Brianna Hildebrand would be playing Negasonic Teenage Warhead in the Deadpool movie, the general reaction among fans was, "Who the heck is Negasonic Teenage Warhead?" That's an understandable response given that Negasonic Teenage Warhead's entire comic book history is contained in a few pages of a single issue New X-Men (and a few issues of Astonishing X-Men, but that turned out to be a psychic illusion subconsciously projected by Emma Frost, so it doesn't really count).
But it looks like Negasonic Teenage Warhead is getting a second lease on life in the Marvel Comics universe. Deadpool and the Mercs for Money #1, by writer Cullen Bunn and artist Iban Coello, introduces a new and different Ellie Phimister. This version of Negasonic was first teased in Marvel Now! Previews on the promotional artwork for Deadpool and the Mercs for Money #4, suggesting that she will eventually join Deadpool's team, but that's not where she starts off.
In Deadpool and the Mercs for Money #1, Deadpool and his team have been hired to find people with powers tied to radioactivity. This leads them to the home of a teenage girl, and that teenage girl is…
That Deadpool finds Ellie while tracking radioactively powered people makes it a little unclear if Ellie is still supposed to be a mutant. In the past, radioactivity has been one explanation (among many) used by Marvel to explain mutation (hence the "Children of the Atom" moniker), but Marvel has pivoted more towards evolution as an explanation for mutants over the years.
Deadpool sends his Mercs inside Ellie's house to capture her, but she uses her precognitive powers, which are more in line with her previous comic book appearance than the explosive powers she wielded in Deadpool, to incapacitate the Mercs one by one. She only stops when she runs into Deadpool and her powers show her a vision that she wasn't expecting.
So it seems like Deadpool is destined to save Ellie from a dire situation sometime in the near future. However, it also seems like Deadpool is the one who putting Ellie in that situation, to begin with. Deadpool hands Negasonic Teenage Warhead over to the scientists who employed him and talks himself into believing their line about trying to protect Ellie and others like her, but the issue's final page certainly suggests their intentions are less than benevolent.
I guess now we know where Negasonic Teenage Warhead in the comics gets the buzzed hairdo that Hildebrand had in the film and that she has in the teaser art for Deadpool and the Mercs for Money #4.
It is still unclear how this version of Negasonic Teenage Warhead relates to the one from New X-Men, who supposedly died when Sentinels razed Genosha. Their power sets seem similar – Ellie dreamed of the Sentinel attack before it happened – but it is unclear if the new Ellie is actually a mutant and how she's now alive after Genosha or what happened to her greenish skin. After Ellie agrees to go with Deadpool, the house she was staying in is revealed to be a psychic construct of Ellie's own creation and fades away. Perhaps these illusions have something to do with her confusing continuity. Alternatively, Marvel did just recreate their entire universe during the events of Secret Wars, so perhaps that's as much of an explanation as we can hope to get. If we do learn anything more, we expect it will be revealed in the ongoing adventures of Deadpool and the Mercs for Money in the coming months.