He first appears when he sees Chrome Dokuro while taking a walk and stalked her since then.
During the gathering of Tsuna's Family and the Simon Famiglia on the rooftop of Namimori Middle School, he was the only one to be away from the gathering. He was grouped with Chrome to patrol other areas near Kokuyo to protect Tsuna. He secretly gave Chrome a package of food with the hope that it would feed Chrome, since he knew that Chrome was only eating chocolate snacks and commented to himself that he had a "kind-hearted soul." Later on, when Adelheid discussed with the rest of the Simon members whether they were going to protect the Vongola Famiglia, Julie was the first to decline, saying that he didn't feel well enough. Adelheid then asked everyone if they have been taking good care of "that," to which everyone responded with a yes, and asked Enma to show them the Ring that proved that he had the inheritance to be the Boss of the Simon Famiglia. When the Simon Famiglia agreed to protect Tsuna, he introduced himself to Chrome and revealed to her that he was the one who kept sending the food for her. He then asked her to go on a date with him. Unfortunately for him, Chrome didn't understand what he meant and refused him, leaving Julie to comment on her cuteness.
He was later seen at the Inheritance Ceremony with the rest of the Simon Family. He and the rest of the Simon were all surprised to see Takeshi Yamamoto healed from his wounds, but didn't say anything about it. He was later seen amongst the crowds during the ceremony, waiting along with the rest of the Simon Famiglia, when Tsuna was to receive the Vongola Sin from the Ninth; however, before the Ninth could give the Sin to Tsuna, an explosion was heard.
After an explosion, the Sin was broken, supposedly by Enma. The Ninth revealed that the Sin in the ceremony was merely a fake. He proclaimed that the real one was in a safe that was impossible to penetrate. Suddenly, the safe was broken open, revealing Julie and Enma with the rest of the Simon. Enma states that they had come to the ceremony to take back the "Sin" for the Simon Famiglia and to get revenge on the Vongola. Enma and Adelheid explained the story about Sin, and that it was the blood of the first Simon Boss, Simon Cozzato. Timeoteo, however, states that he had never heard this story before, and that the Simon Famiglia had disappeared overseas on their own accord. Julie replies that he was expecting that answer and that they had never heard of that story. Julie and the others then released their rings as Enma said that the reason that Vongola Primo got rid of the Simon was because they had a power that rivalled the Seven Flames of the Sky, which was the Seven Flames of the Earth. Enma easily overpowered Tsuna and defeated him and his guardians. Julie takes Chrome and says that "we have date plans" while Tsuna watches. The rest of the Simon Famiglia then left to go back to their home.
He later reappers with the rest of the Simon members, when Tsuna and his Guardians arrive to save Chrome and greets them. Tsuna and the others ask if Chrome is safe, Julie quicklys replies that she has been "asleep on my bed, looking as cute as an angel". Just than, the Vindice interrupt them and state that they were there to complete the oath between the Vongola and Simon Primo. They go on to say that if the two families ever cross paths with one another, the loser would have spend the rest of their lives in the Vindice Prison.
The Vindice take their leave, but before going, Adelheid ask how would they would know who wins the battle. The Vince reply that if the loser's pride is broken, than the other Famiglia wins. Enma tells Tsuna and the others that since Simon Island was their home land, they wouldn't engage in battle yet. They would have to follow one road that leads to them. The Simon then leave.
He later received the memory of Cozzato and Giotto when they first met. During that same period, he watches, with a grin on his face, as Chrome awakens. He then approached her and explained that he had took her with him when she was unconscious at the Inheritance Ceremony and right now she is at Simon Island, inside his room and also told her that Tsuna and the others currently in the island to save her. He then offered to help her change her clothes, which Chrome immediately refused. Still, he insisted because of her cuteness. Fortunately for Chrome, Adelheid came and took Julie to Simon Ring Holders' meeting.
On the way, he teasingly asked Adelheid if she was jealous, which angered her. She told him that he is not supposed to be playing around and that she would kill Chrome after they have no use of her, much to Julie's protest. When they arrived at the meeting place to speak about the memory they received after Ryohei and Kouyou battle, Julie simply said that he has no interest in that. As he leaves Adelheid attempts to remind him of his duties as a guardian of Simon. Julie, taking no delight from those orders, grabbed her face and glared at her coldly, reminding her of his displeasure of being ordered around. Then he playfully said not to worry because he will help if things get bad and he never forgot their goal is to overthrown the Vongola.
After the battle between Large Ooyama and Lambo, he receives the memories. He then tells Kaoru Mizuno that he need not battle since he had already defeated Yamamoto. He then proceeds to volunteer himself to battle the Vongola next on, which Shitt P! takes a leave. He then settles down beside the depressed Enma. He tells Enma that their spy in the Vongola's CEDEF is doing well and is getting a lot of information useful to themselves. He takes out the picture of the leader of the CEDEF, Iemitsu Sawada, to prove that the "dirty" Vongola blood runs throughout Tsuna. He seems to have successfully manipulate Enma into continuing the fights. It was then revealed that Julie has a secret agenda for the Simon Famiglia when he manipulates Enma.
When Enma comes and tries to save Shitt P! (which fails) and challenges Tsuna straight, he slyly tells Adelheid that she should leave Enma alone.
As Enma is in pain because of the gauntlet's evolution, Adelheid postpones the battle and declares the rules for the next battle, Adelheid against Tsuna, for a battle of pride. Gokudera disagrees of this because of her making the rules on her own, but Julie is quick to say that their lives were simply "spared a little longer" then saying that Enma would become much stronger after his awakening Adelheid then calls for Julie as they take their leaving, but not before Julie says, "just like Primo".
As Chrome is trying to escape the room she is held captive in, the door opens to reveal Julie, and he starts to question her. He then asks if she could be his, only for Chrome to reply "no", but Julie says that she and him are getting along more than Mukuro Rokudo. Julie then begins telling Chrome about how Mukuro is keeping her alive with his illusions, but there is a special barrier in the island making him unable to reach, or even use his illusions for Chrome. Chrome is surprised by this, but even more surprised when Julie says that he is making her organs now, Chrome is unbelieving of this, until Julie gave an example by removing her organs. Julie then gives them back to her and asks if she will be his, she once again says that she won't, and that she doesn't need his organs. Julie tells Chrome that he was the person who devised the plan of Vongola vs. Simon, and of his two goals; first, to destroy Vongola, and second, to take Mukuro's body. As he says this, he reveals his true form as Demon Spade. Chrome refuses the offer, and Demon is forced to use his eye technique as Chrome is put under his control, forcing her to obey his will.
When Demon watched Adelheid's fight from afar, it was revealed that he used Julie as his vessel to revive the Simon Famiglia's power. After Demon was supposed to be defeated by Mukuro, Julie was freed from Demon Spade's control and had no recollection of what happened during possessed by Demon, claiming that he feels he has experienced long dream. He protected Enma when Demon intended to send them to his illusionary world, causing him to be sent there along with Gokudera, Yamamoto, and Lambo as he watched Enma, Tsuna, and Hibari fight Demon. Once Demon was defeated, Julie and the others who were trapped in Demon Spade's illusionary world was freed.
A week after the battle, Julie, Kouyou, and Ryohei were captured by Adelheid for breaking school rules, Julie was captured for flirting with female students, much to Adelheid's jeaolusy. He was furthered punished by Adelheid when he attempted to flirt with Kyoko.
Julie is first briefly seen dissuading Adelheid from "taking care" of Skull, stating that he hated diaper duty.
Later, when Adelheid stumbles upon Enma and Skull after nearly getting killed by Team Mammon on the first day of the Representative Battle of the Rainbow, Adelheid thanks Skull for saving Enma and agrees on behalf of the other Simon Guardians to become part of his team in the Representative Battle. Thus, Julie joins Team Skull.
On the second day, no one challenges Team Skull until just before the battle ends, whence a group of Vindice that eventually become known as Team Bermuda appear and defeat all of Team Skull in an instant, stealing their Rainbow Wristwatches to enter the battle as well. Team Skull is mortally wounded, but still alive at this point.