After the exile of François-Thomas Germain, she became the informal lieutenant of her husband, mediating between his conservative views and the extremist faction which aimed to radically reorganize the social order in France.
When Élise was five years old and entered the convent, Julie and François visited the Mother Superior. There, Julie caught a glimpse of her daughter's scars left by the Mother Superior's caning. Angered, Julie demanded answers from the convent leader and threatened her, which ended up with Élise leaving the convent.
Unbeknownst to her husband, Julie secretly introduced Élise to her former lover, Frederick Weatherall, a British Templar, on a winter day in February 1774. She lead her to the grove at the perimeter of their château's south lawn with their dog, Scratch, and warned her to not be alarmed if she saw a gentleman in the shadows. Aware of Weatherall's presence, Julie taught Élise to be aware at all times and how to search for a man's whereabouts or clues. The young girl failed to find the hidden Weatherall and Julie asked him to reveal his presence behind a tree, exclaiming that she had seen his shadow, much to his amusement.
After assuring Élise that he was a confidant and protector, they talked about the King and François' plans. A growl from Scratch interrupted their conversation and they found themselves watched by a wolf. Concerned for her daughter's safety, Julie immediately swept her away and took out a blade from her muff. Weatherall restrained Scratch and reached for his sword's hilt, but he was stopped by Julie, who him that the wolf was aware of their strength and they better let it retreat.
Julie's predictions were correct and the wolf eventually retreated, leaving a parable for Weatherall and Élise in awe. Julie asked him to train her daughter in the art of sword-fighting in secret, which he was happy to oblige; this was also kept as a secret between the three of them from François. Much later, Élise questioned her mother about the blade and Julie told her it was for protection against "four-legged" and "two-legged" wolves. This topic became one of their vérité cachée, secrets.
During the spring of 1774, the Carrolls, a British Templar family, visited the de la Serres due to their concerns of François' plans for keeping France in order. Julie and Élise were tasked to entertain Mrs. Carroll and her daughter, May, in which they invited the latter for a walk. Mrs. Carroll told Julie about the London Templar's concerns for the intentions of François wherein they argued about change and loyalties. Julie assured her that she would abide their principles and would not allow her husband to be swayed by his advisers.
Two years later, Julie and Élise went shopping for shoes in Christian's shop located in Paris. After arranging their payment and delivery, Julie noticed that their carriage and coachman were missing. She became suspicious, although she assured Christian and her daughter that everything was alright and that their coachman was merely late. Christian warned her that the people were dangerous and it was beginning to get dark. Tense and watchful, Julie led her daughter along the street and told her to be calm as someone was indeed following them. The pair kept walking and stopped to look into another shop window, finding that their stalker had disappeared.
They took a narrower alley, lit only with lanterns at each end, to reach the square where carriages could be hired. They were stopped by their stalker, who drew a blade from his doctor's bag. Julie tried to go back but a lamplighter blocked her way, who doused the candle and reached for a dagger. Finding that their stalker was an Assassin, she cut him with her knife and killed the lamplighter with a footblade after making an attempt to murder Élise. The Assassin fled and Julie took the doctor's bag that he had left. Not long after they had arrived in their home, François and his Templar associates fussed over the pair, but Julie did not tell them the whole story and kept the fact that their attacker was an Assassin hidden.
Unable to hide the Assassin-Templar war from Élise after the incident, Julie and François revealed her destiny and told her she would be trained by Julie and Weatherall in combat and beliefs by her father. Their secret was revealed, and François asked Julie if she still had feelings for the British Templar, to which she replied that she was embarrassed. Soon after, Julie arranged a meeting with Weatherall, revealing to him that their stalker was a British Assassin through the doctor's bag. She suspected that the Carrolls were somehow behind the incident and wanted her dead; he promised her that he would investigate the matter.
Later that year, the de la Serres adopted Arno Dorian who became Élise's playmate. François and Julie told their daughter about Arno's past and nature as an Assassin. Although the boy was oblivious to this fact, François insisted that Élise bring him into the ways of the Order, an idea to which Julie was opposed. She asked her husband to delay Arno's indoctrination since the children were still young.
Julie fell ill soon after their discussion and was confined to her room. François and Élise spent their time visiting her in bed. Weatherall also visited her and cried by her bedside. Before she died, Julie asked Élise to stay strong and to accept her death as God's purpose. She asked her daughter to be the voice for François and to lead the Order by Élise's own principles. She reminded her that she would always be proud of her and that she loved François and Élise deeply. Julie died on 17 April 1778.
Personality and characteristics
Unlike the other women at court, Julie was not obsessed with looks, status, nor gossip and had always kept a respectful distance from them. She had no time for beauty and fashion, with the exception of Julie's fondness for shoes. The attention she gave for her comportment was only for her to maintain decorum.