Negasonic Teenage Warhead (real name Ellie Phimister) is a fictional mutant character in the Marvel Comics universe who first appeared in New X-Men vol. 1 #115 (2001). She was a student of Emma Frost. An illusory Ellie appears as a member of the Hellfire Club in Astonishing X-Men. The 1995 Dopes to Infinity album by Monster Magnet contains a song called "Negasonic Teenage Warhead", presumably the source of her name.
Carrying what appeared to be Ellie's corpse, Emma Frost, who had survived the Genoshan genocide thanks to her secondary mutation that turned her into diamond, was found by the X-Men's Beast and Phoenix (Jean Grey). Emma proclaimed the teenager, who had named herself Negasonic Teenage Warhead, to be a credit to her family and the mutant race and then suffered a mental breakdown when she found out Ellie was dead.
Much later, Negasonic Teenage Warhead was apparently seen to be alive as a member of the latest incarnation of the Inner Circle of the Hellfire Club of New York, alongside Cassandra Nova, the Black King (Sebastian Shaw), Perfection, and Emma Frost.
The new Inner Circle attacked the X-Men at the Xavier Institute. While Shaw, Frost and Nova dealt with Beast, Wolverine and Colossus, Ellie "dreamt" that Shadowcat was phasing down to the Earth's core and was unable to stop, which in turn came true.
Later, a comatose Cyclops woke up and began shooting the members of the club, including Ellie, with a pistol. It was revealed, however, that Ellie and all the other members of the Hellfire Club were projections of Emma Frost's mind; these projections disappeared after they were revealed to be illusory.
After hearing Negasonic Teenage Warhead's codename, Kitty Pryde remarks, "Wow, we really have run out of names."
Astonishing X-Men vol. 3 #15 reinforces that Ellie has precognitive powers when she claims that she dreamed that Kitty was phasing down to the Earth's core which in turn came true, but as this Ellie was a mental projection of Frost, it is possible that this was just a manipulation of Kitty's mind on Frost's part. Cyclops states as much in-comic. All of Frost's other projections acted as if they had the same powers as the mutants she was impersonating, so this supports NTW having strong telepathy (including mind control) and precognition.