Julian Keller was leader of the Hellions, the Xavier Institute training squad instructed by co-headmaster Emma Frost. His young age notwithstanding, Keller has already learned to employ his genetic mutation in a variety of inventive ways, including flight and concussive force blasts as well as the creation of telekinetic shields and transport platforms. Always looking for trouble, he was involved (along with Josh Foley) in reviving Magma and nearly incinerating the medical wing, as well as turning away Noriko Ashida Surge II) because she was dirty. He was originally an advisee of Northstar, but his constant punishments caused him to switch advisors to Emma Frost, who sees in him great potential for heroism as well as a frightening disposition for personal gain and villainy, Julian named himself 'Hellion' in tribute to the former White Queen's first class of students (who had been killed by Trevor Fitzroy). Hellion has demonstrated quite a significant interest in Sofia Mantega, the half-Venezuelan co-leader of the rival New Mutants squad. He is particularly hard on David Alleyne, the New Mutants' other leader, whom he views as possessing a particularly useless mutant power and as a genuine competitor as a leader and strategist. However, for the school dance, he dated all three Stepford Cuckoos.

True to his namesakes, Hellion is hard-pressed to stay out of trouble. He led his squad to try and rescue Kevin Ford (Wither) from FBI custody, but they were stopped by the New Mutants squad before they could interact with the government agents. Julian has a major crush on Sofia (Wind Dancer) though they lead rival squads. Hellion is arrogant and vocal about it, but he does possess a talent for leadership, as evidenced by his squad's winning of the first school Field Day prize.

Following the end of school, Julian invited his squad to his parents' house in California. Getting a boy made of rock and a girl made of metal onto a plane proved difficult, and Hellion started a minor brawl with airport security. After standing down, he called his family lawyer and got the Department of Homeland Security to let them on the plane. On arrival at his house, however, his parents were disappointed in the public embarrassment he caused and effectively disowned him. Julian was livid, and recalled that his parents' wealth happened very suddenly, so he broke into their safe and found documents about a "Kingmaker," and the Hellions summoned him.

The Kingmaker arrived and agreed to grant the Hellions one wish. Julian wished for fame and glory, and soon was in a position to save the Governor of California from a sniper. In return, the Hellions were asked to bring the Kingmaker a biological weapon from Genetassist, Inc., which he planned to give to Dr. Octopus. Hellion led the raid, taking out Diamondback and Paladin in the process, and retrieved the canister. However, Hellion only agreed to give it to the Kingmaker in return for voiding their contracts. When the Kingmaker agreed, Hellion attacked, and with his full squad managed to defeat the villain, in the process also destroying the biological weapon before turning it over to S.H.I.E.L.D.

Following the advent of "M-Day", all of the students still with powers were forced by Emma into an all-out brawl, and the ones deemed to be the best were assigned to become a group of in-training X-Men. Julian was among those students that made the grade. Recently, after hearing about the death of their teammate Icarus, the New X-men took it upon themselves to destroy Nimrod once and for all. During the fight with Nimrod, X-23 was severely injured and was unable to heal herself. So Julian decided to fly her to the mansion, he knew that he would not make it in time, so he contacted Emma Frost mentally and asked for her help. She helped him by removing the mental blocks in his mind that kept him from using his powers to his full potential. Because of this Julian made it to the mansion in time to get Elixir to heal her. The removal of the mental blocks caused severe physical injuries however, and he was last seen asleep and watched over by X-23, who left seemingly hurt when a still dreaming Julian spoke Sofia's name.

After Julian regained his strength, Beast brought Julian into the empty Danger Room to test the limits of his powers after getting the mental blocks removed. Julian accidentally blew up the floor of the Danger room, while picking up a paper clip. Beast suggested that he practiced away from the students until Marvel Girl returned from space. After speaking with Beast, Julian began to search for Cessily, who had been gone with Laura for a long time. When entering the room of Cessily and Dust, Julian saw Laura, who was preparing to leave the mansion to get her. Though Laura was reluctant, Julian came along with her. After finding a hideout for the facility that made X-23, Julian watched Laura interrogate a man at gun point who may have known where the facility was. After Laura got the information she needed she shot the man. Julian argued with her by saying that, if they were going to look for Mercury, they where going to need a different way of interrogating.

Recently, Julian was severely wounded during an attack on the X-Mansion and later, also,the New X-Men disbanded.