|“|| Is petty larceny the extent of your ambition? Have you no mind to comprehend the scope of ours? All the empires on Earth, abolished! A free and opened world, without parasites like you! (May the hell you find be of your own making).
Julien first appears in In July 1715, when he, Woodes Rogers first meet Duncan Walpole, a former assassin who was defecting to the Templar Order and arranged a meeting with Laureano de Torres y Ayala, the Grand Master to be inducted into the Templar Order. While he was staying at the governor's residence in Havana, du Casse and Rogers found themselves waiting longer than expected, as the man they believed to be Duncan Walpole arrived a week late. In actuality, the man that joined them was a pirate named Edward Kenway who had assumed his identity in order to claim Duncan's promised reward.
Following the introductions, du Casse, Rogers and Kenway passed the time with a shooting session, as They were were still waiting to meet with Torres. After Edward beat Rogers' shooting challenge, du Casse was surprised that he didn't have any Hidden Blades. Edward replied that they were damaged. Du Casse gives Edward a new pair that he got from some dead Assassins.
Before meeting with Torres, du Casse and Rogers suggested that "Duncan" show his skills in assassination techniques. Edward was able to do as he had free running abilities and was able to mimic these skills very well. The three then went to meet with Torres, who, after inspecting the blood Edward brought with him, led them inside, where he officially inducted the three in the Order.
Torres told them his plans of locating the Observatory, with the help of a Sage and pirate named John Roberts, which would help the Templars' goals of a New World Order. Du Casse became skeptical to the Sage's credibility, especially it had been over 40 years since a Sage was around. However, Torres reassured him they were certain. After toasting their schemes, the group of Templars made arrangements to convene the next day at the Castillo de San Salvador de la Punta and then turned in for the night.
The next morning, du Casse and his Templar allies met the captured Roberts, and mentioned that a Sage's blood was required to enter the Observatory. While escorting Roberts to Torres' mansion, the Templars managed to fend off an Assassin attack, during which the Sage was nearly able to escape. With Rogers leaving for England, the other Templars decided to imprison Roberts and interrogate him the day after.
However, Roberts managed to escape later that night, leaving a trail of bodies in his wake. When the Templars arrived at the prison, they found Kenway instead, who had just arrived with the intention of interrogating Roberts himself. Realizing he was an imposter, the Templars apprehended Kenway and sent him to be taken with the Spanish Treasure Fleet as a prisoner.
du Casse eventually earned Torres' trust and was tasked with ensuring a constant flow of weapons, ammunition and gold to him and he became appointed captain of one of Torres' largest galleons, El Arca del Maestro, which du Casse would frequently dock in the natural cove of Great Inagua, his island hideout. Unknown to du Casse, Edward escaped his imprisonment aboard the Spanish Treasure Fleet, and later sought to acquire El Arca del Maestro, so that it could be used as a defensive measure for the pirate republic of Nassau. To accomplish this, and prevent news of his survival and escape from getting back to the Templars, Edward infiltrated the island and bypassed du Casse's guards. While du Casse was busy preparing for another voyage, Edward snuck onto the vessel's rigging and ambushed the Templar from above
As he was dying, du Casse berated and taunted Kenway for his small ambition and ignorance, and called him a "parasite". The pirate ignored these words and took the key that was in du Casse's possession. Refusing to tell Kenway about the key's function, du Casse cursed the pirate, before he died.
Kenway would later took du Casse's hideout for his own, renovating the village and manor, and collected four other Templar keys to gain access to a special set of armor.