Julie's mother had been one of the first Shadowhunters to be forcibly Turned into Endarkened soldiers by Sebastian Morgenstern. Afterwards, she, along with her father, her sister Elizabeth, and the other residents of their Institute, evacuated to Alicante. During the last night of the Dark War, Julie was among the children locked up in the Accords Hall for safety. However, the faeries of the Wild Hunt unexpectedly made it inside the building. Elizabeth was one of the many casualties, and Julie witnessed the very moment the faerie, whom Julie noted was beautiful and had silver hair, struck down her sister with his sword. Julie was left with her father who, for some reason, Julie considers useless.
Traumatized by the events of the war, Julie decided to attend the newly-reopened Shadowhunter Academy "to learn to fight," as opposed to just training in Alicante or at their Institute, so she would be able to do something the next time she's in battle.
Julie spent most of her time at the Academy with fellow Shadowhunters Jon Cartwright and Beatriz Mendoza. She also befriended mundanes George Lovelace, under the impression that he was a Shadowhunter by blood before he revealed otherwise, and Simon Lewis, due to his hero status among those aware of his contribution to the war, while the two were still in the elite stream alongside her. After George and Simon switched to the mundane stream, she became much less friendly towards them,  though she eventually became good friends with the two.
When Julie was included in the group of twenty students, considered the best and most promising in their class, that would handle a Downworlder problem in New York, Julie immediately became concerned. She was reminded of the war and of the situation with Downworlders and was desperate enough that she convinced Jon and Beatriz to approach Simon for advice. When Simon insisted that he truly lost his memories and had little idea about Downworlders, Julie opened up to him about her experience during the war.
After they returned to the Academy for their second year after the summer vacation, Julie had begun dating Jon. When Helen Blackthorn, a known half-faerie half-Shadowhunter, came to the Academy to give a lecture, Julie walked out of the class without saying a word. She waited outside the classroom for Jon until he came out with Beatriz, George and Simon, who were having a conversation about Helen's lecture, and answered George's rhetorical question, saying she already knew Faeries were evil. After Simon started talking about how wrong the treatment Helen got was, Julie coldly reminded them about her sister's death and declared that she didn't care about Helen or her "so-called feelings", before her voice started shaking and she abruptly slit from under Jon's arms and raced off down the hallway, with Jon soon taking off after her.
Along with Beatriz and even Jon, she had become somewhat friends with Simon, George, and Marisol and was much more civil with them once they had adjusted to being trained with mundanes, or "dregs", despite their history. The six teamed together for their training missions and Julie succeeded as team leader, despite George apparently almost screwing up the mission with his hesitation. George has also implied that Julie liked Simon, though the latter was unsure if he had been referring to Julie or Beatriz.
In 2012, Julie attended the war council meeting called by Julian Blackthorn at the Los Angeles Institute, where she stood and tried to defend the Shadowhunters whom she didn't want to hurt in the battle against the Cohort. Julie also proclaimed that she was close to Jace Herondale—as she used to at the Academy—while under the impression that Jace was dead, and when the alive and well Jace revealed himself to the crowd, he called her "bestie".
Soon after, she and Beatriz joined the procession of Shadowhunters at the parley staged by the Cohort. Julie was struck by Oban's arrow, which was meant for Kieran, and died, making her the first Shadowhunter slain in the battle.
Personality and traits
Julie used to hold a generally negative view of mundanes and Downworlders, as was typical of most Shadowhunters—until she met Simon, who tried and succeeded in swaying their views of them—though she had a particular hatred towards faeries whom she blames for her sister's death. She was often cold and distant, though was also capable of fear and being open about it, and was also able to eventually develop a friendly camaraderie with those she trusts.
Julie, also like most Shadowhunters, moved with easy grace. According to Simon, she was also highly-skilled, even during the early years of her training.
Julie was a pretty girl with features that went with her narrow face, making her expression often appearing pursed with disapproval. She had silky blond hair, porcelain skin, and gold-specked hazel eyes that seemed to change color depending on the light. She was also tall and curvy.