In the beginning they had an awkward tension-based start, Lacie had revealed that Jack was afraid of Oswald's gaze. However, Levi noticed that Oswald would go out to meet Jack even though he outright claimed that Jack made him feel uneasy. Upon Levi's surprise that Oswald would go with Jack despite their tense start, Lacie corrected that Oswald likes and is interested in Jack. After spending time with each other they became best friends with Jack being confused, but appreciative that Oswald stayed by his side even though Oswald said that Jack made his skin crawl. Later in canon (after Oswald took the title), they made a pocketwatch for the two of them with Oswald's composition, 'Lacie', and spent a good deal of time with each other. Glen confessed talks four times as much when he's around Jack; which, looking at the few words since, seems to mean that he is very introverted.Jack holds Oswald in high respect, as he is Lacie's brother, and because Oswald became his best friend he also became second to Lacie in terms of importance to Jack. Jack's world is a world that revolves around Lacie and Oswald, his key obsessions; but, he is faced with Miranda's unwavering resolution for his dear best friend's head. He seems to have distanced himself from Oswald after Lacie was thrown to the abyss and he asked Oswald to go together to retrieve Lacie, but Oswald harshly refused. Jack couldn't accept Oswald's rejection and began thinking that 'Oswald' is dead and replaced by 'Glen', even though in front of him Jack smiled and apologized for saying something stupid.
After Jack tried to sever the chains of the world, they became enemies. Oswald's personality seemed to change to be more cruel after feeling betrayed by Jack. Though Jack claimed he thought Oswald as a friend, he shared his confusion as to why he wasn't sure of that, for he did not hesitate to kill him because Oswald was trying to stop his plan. Jack considered 'Glen Baskerville' as his biggest obstacle to Lacie.
When Oswald's soul was fading due to his abuse of Leo's body, Jack stood and watch the spectacle silently, then cried. Upon Oz's question whether he was sad, Jack answered he didn't know, but tears wouldn't stop.
Jack had a complicated relationship with Lacie while she was alive. Although Lacie was the one to show Jack that there is light even in the greatest darkness, Jack's opinion of her differs greatly. In truth because Jack's existence is hollow and reflective, Jack was confused about his identity, and it was only through his interactions with Lacie that he was able to identify his feelings and thereby his "true self". For this reason, Jack became obsessed with reuniting with Lacie in order to satisfy such a feeling once again. Even so, during his ascent in the social world, Jack grew to despise Lacie for she was bounding him to this world, as he was aware that there was no way he could carry on living without her. However, he came to realize that his hatred and fear was his true self, and that Lacie was the only one capable of provoking such emotions in him.
After reuniting with Lacie Jack was overjoyed, and although his obsession initially appeared to be rooted through romanticism, Jack still detested Lacie - having divulged this information to Levi on one occasion. As well, Jack drew himself a boundary which he dared not cross in relation to Lacie, never touching her or making any form of progression toward her - something which caught Lacie off guard and ultimately led her to develop some romantic feelings for Jack.
Jack's contentment after his reunion was rather short-lived however - entering a state of empty misery after he'd learned of Lacie's death. Even though Jack despised her, he needed her, which is why he remained rather unattached following her death until Levi came and told him of Alice and the Chains of the World. With the information Levi provided, Jack formulated a plot that led to the Tragedy of Sablier, having intended on reuniting with Lacie once more - as he believed that she had become one with the Abyss rather than dying and he wished to deliver to her the world she loved so much. After his failure, Jack didn't stop his pursuit, plotting over the course of 100 years in order to once again attempt to plunge the world into the Abyss for his own personal satisfaction of identity by once again reuniting with Lacie.
It seemed that Miranda Barma helped Jack in his pursuit to contact the Baskervilles. In return, Jack commits himself to a bargain with Miranda Barma, who tells him of Oswald; it is later revealed by her that Oswald's head is the price for gaining society, and therefore Lacie. He finds himself at odds with Oswald when he attempts to retrieve her, and the process of revelation is still unfolding as to the events of the Tragedy.
In his notes, it was revealed that Arthur once admired Jack for being smart, kind, and handsome. Meanwhile Jack used Arthur to get his sister, Miranda. Jack told Arthur the lie of the tragedy of Sablier, thus on his behalf Arthur wrote down in his diary. However, after hearing the real truth through a seemingly unbelievable tale by Jack, Arthur was terrified of him. He was lost, fearing if that Miranda would be arrested if he revealed the truth, but couldn't bear the truth alone. Arthur then wrote the truth in a secret section of his diary, hoping someone would find out one day. Arthur died due to a high fever after finishing his notes.Tragedy he had a pretty good relationship with the Intention, the white Alice, and was always cheerful with her, but the black Alice disturbed him, and he was easily exhausted when with her. Black Alice was mad and Oz ' broke in his hands after taking in the darkness that was destroying Lacie's memories.
He was first introduced to Alice through Levi, who revealed both his accursed state and the part he played in helping create her.When Oswald refused to reveal anything about Alice, Jack pretends that this was the first time he'd been introduced to her. He tries to get Oswald to tell him her name, but he retains it. He also realizes that Oswald did not know much about Alice himself, and rather than search for the answers, he was trying to stay out of it. He then asked a surprised Alice whether her name was what Oswald refused to tell him.
Jack soon developed a close relationship with the white Alice, caring for her as a friend, developed a strong bond and teaching her. Eventually, though, he convinced her that he wanted to make the two worlds come together by breaking the chains that held them apart, and that it would make him able to see the 'real' Alice, because she was using the other Alice's body. She draws him a picture of a rabbit chain that she believes will be perfect for him, but then admits she cannot remember his name.
Gil and Jack had a strong relationship. Gil is the first person who found the piece of his soul within the warp in Sablier, in which Jack says that he simply wanted Gil to have eyes for his new master, Oz, but said that he was so happy he had called his name again. It is later revealed that Jack saved Vincent and Gilbert from an almost routine, cruel beating, on account of Vincent's red eye, which reminded him of Lacie. He took them to his family's mansion, and after learning of their situation, informed Oswald, who then became Gilbert's master. Although Gil and Vincent came to visit him every so often, Glen kept a close eye on them. Even though Jack called the siblings his "cute little friends" it is implied that Jack favored Vincent over Gilbert. However it is also hinted that Jack feels guilty for betraying Gil as in chapter 65, Jack cried when he wounded Gil and in chapter 19 as Gil cries in pain for not being able to recover his lost memories, Jack pats his head and apologizes to him. In chapter 22, while Gil struggles with unpleasant memories, Jack pats his head again. However, after Gil learned the truth and remember his past, Gil declared that he can never forgiven Jack for what he done.
Jack and Vincent had a very close relationship. Vincent called him master even though Jack told him that it was unnecessary, and even after Jack tried to persuade him that nicknames would better suit him, Vincent persisted on calling him "master". Jack was probably the closest person to Vincent next to Gilbert, and their relationship can be compared to brothers (Jack told Vincent to call him onii-san). Jack rescued Vincent and Gilbert from being beaten, and after learning as much as he could about them, allowed them to be taken to the Baskervilles. According to the confession written by Arthur Barma, Jack was only using Vincent as part of his plan to plunge the world into the Abyss. He was however shown to be somewhat attached to Vincent, likely due to being a Child of Misfortune like Lacie, and was more than likely uncomfortable with Gilbert becoming the next Glen.'It was the same thing... all over again.' It is also hinted that Jack had an attachment to Vincent and Gil, as he took both to meet Alice so they could play together, and during the Tragedy of Sablier, before the confrontation with Glen, he is seen carefully carrying little Vince in his arms.
Vincent had also refused to believe Jack's involvement in the Tragedy of Sablier, implying that Vincent had idolized, even respected, Jack.
Like Lacie, Jack's relationship with Oz was complicated. Originally, Oz was a chain that was created by Lacie during her attempts to end the Core of the Abyss's loneliness. After the Core embraced Lacie's daughters while Lacie was destroyed from the darkness of the Abyss, Oz became acquainted with her daughters: Alice and the Intention of the Abyss. When the Intention recognized Jack's voice through Alice's body, she asked Oz to give Jack a piece of Lacie's thoughts and feelings, believing that Lacie wanted Jack to know her true feelings for him. Oz accepted this request due to his desire of wanting to thanks Lacie for bringing him to life. He also saw from Lacie's memories that Jack was someone important to her. Although Oz was aware of the fact that his body will be destroyed after accepting the small fragment of Lacie's soul, due to the darkness of the Abyss trying to destroy all of Lacie's presence, he was succeeded into giving Jack the fragment of Lacie's soul. However, Oz was unaware of the fact that his actions of giving Jack Lacie's thoughts and feelings toward Jack was a horrible mistake.
When the Intention transform Oz's body (his other body that remained with the Intention) into the chain B-rabbit, Oz once again met Jack after Jack tricked Vincent into opening the Baskervilles's Door to the Abyss. When Jack asked Oz to form a contract with hm, Oz remember the reason why he gave Jack the fragment of Lacie's soul was due to his wish of wanting to repay Lacie for giving him his life. Despite his wonders and suspicions of whether his latest action of giving Jack the fragment of Lacie's sould had created a misunderstanding, Oz accepted Jack's request and formed a contract with Jack, before proceeding to destroy the chains wrapping the world.
Oz's worst suspicion was confirmed as soon as Jack mercilessly order him to slaughter the Baskervilles who came to aid Glen, as well as injuring and finally beheading Glen. Unknown to Jack, Oz was horrified of the actions he just committed. His cries and sufferings was heard by Alice who was inside the tower. After Alice confirmed her doubts that Jack was the one responsible for Oz's anguish, she tried to have Jack sympathize with Oz only to have him spited his indifference towards Oz and become enraged before trying to make her switch places with the other Alice. When Alice committed suicide in order to protect those she love - Alice and Oz - Oz could only stare in shock and was in dismay over the fact that he couldn't stop Jack from hurting Alice. He was severely depressed. Later on, Alice (in spirit) took over Oz's body, taking his powers as B-rabbit and his memories and released him from Jack.
After the events of the Tragedy of Sablier, in which Jack took advantage and manipulated the truth, Oz remained dormant in Jack's body. After Jack convinced the current head of his family, Xai Vessalius 100 years later and the last pieces of Jack's soul has disappeared from his body, Oz was woken from his slumber and took control of Jack's body. Because Alice had taken away his memories, Oz was unaware of the truth about himself. When Jack discovered that Xai knew the truth about what really happened at the Tragedy of Sablier, Jack manipulated Xai into believing that by casting him and Oz into the Abyss using the Chains of Condemnation, Xai would be able to get rid of both of them. In reality, Jack was hoping that Xai will cast them into the Abyss, in hope of reuniting with Alice and regaining Oz's powers back.
After returning from the Abyss, Oz saw an image of Jack after the pocket watch fall and one of Alice's memories appeared. Jack told Oz that he's been waiting for him. Jack encouraged Oz to find all of Alice's memories. Jack appeared once more during Oz's attempt to stop Alice from killing Phillipe West's father by telling he can control Alice's power. Oz finally meet Jack during his and Gilbert's rescue attempt to save Alice and Break from Cheshire's dimension. Despite the sudden meeting, Oz believed in Jack when he said he wanted to help him save Alice. Oz, having seen a memory of Alice and Jack together, trusted him and allowed Jack to give him his strength. Jack comforted Oz when he witnessed a scene of the Tragedy of Sablier, as well as when Oz discovered Alice's death scene. After Oz, Alice, Gilbert, and Break escaped Cheshire's dimension, Oz discovered he contained Jack's soul within him during the confrontation with the members of Pandora.
This revelation shocked and surprised Oz. To his dismay, the members of Pandora treated him as someone special rather than a normal person. Oz explained to Reim that it is because they seen him as Jack rather than Oz. Over time, Oz accepted the fact that Jack's soul lived inside him. However, he told Rytas he is just Oz not Jack, showing that Oz see himself and Jack as two separate people. Oz sometimes taken advantage of the fact that he have Jack's soul inside.An example of this is when Oz tricked Isla Yura into believing that Jack took over his body at the moment Oz tried to kill himself and manipulated him into allowing access to his mansion..
All of this changed during the Baskervilles's attack at the headquarters of Pandora. When Oz managed to get Leo to regain his sanity, Jack took over Oz's body and sent one of B-rabbit's chain through Leo's body, much to Oz's horror. While Oz questioned Jack for his actions, Jack tried to convince Oz to use destroy Leo to Oz's refusal. When Oz stopped Jack from harming Leo, Jack was amused when Oz said he is Leo's friend and won't Jack to take control of his body. Jack counter this by telling Oz that's impossible because he knows Oz is trying to denied the memories within him. After telling Oz that his existence is nothingness and that everything he was had diminished, saying that Oz's existence can't really exist, Jack forced Oz to regained his lost memories.
After Oz regained his lost memories, he was completely devastated by the revelation Jack forced him to rememeber as well as the events that really occurred during the Tragedy of Sabiler. During the rescue attempt by Gilbert, and later on by Oscar, Oz was able to regained himself after Oscar revealed his wish for Oz to be happy. This allowed Oz to regained control of his body and freeing himself from Jack's control. Despite Oz hated Jack for everyone he's done, Oz asked Jack to make a deal with him.
During his battle with Oswald (who at the time taken control of Leo's body), Jack once more tried to convince Oz to destroy Oswald in which Oz refused again. During Oz and everyone else's rescue attempt to save Alice and Gilbert from the Core of the Abyss, Jack at first stopped Oz from leaving after the completion of his Incuse. Looking to Oz, Jack asks if it's really okay for him to be doing "it" alone. Confirming this, Oz reveals that he's already said everything he wanted to say to Jack, bidding him farewell as he loosens from Jack's grip and drifts away from Jack's subconscious. This showed Oz, despite his feelings toward Jack, he trusted him for what he's about to do next.
It was revealed that Oz's body was becoming more chain-like as a result of his power and the Incuse on Oz's chest will fully completed his rotation. Regardless, Oz made a deal with Jack which was revealed: to ask Jack to seek out the Intention of the Abyss and rescue her. Oz wanted Jack to tell the Intention the truth of why the events occurred at the Tragedy of Sablier. Oz is aware of Jack's influence on the Intention and know she believed any words Jack said. Intrigued by this, Jack asked why Oz asked for a request and what would Oz do in exchange for Jack's cooperation in fulfilling this request. Oz said he will stop the Chains from being destroyed, which Jack laughed knowing he was the one responsible of the Chain of the World's destruction. However, Oz explained while he aware of this, he will show Jack that his actions was a mistake. Oz reminded Jack about the conversation about the Abyss and Jack's reason for why he wanted to destroy the Chains after Lacie's death. Oz tell Jack that Lacie never wanted the world she loved to be destroyed and that Jack never understand Lacie's wish in the end. Jack rebuffed Oz's statement, asking how can Oz know such a thing. Oz counterattacks by stating Jack never saw the Abyss in its original form. Oz explained to Jack that Lacie not only loved the world she grew and lived in but the other world she saw: the Abyss itself. Jack claimed Oz is lying but Oz immediately told Jack he isn't lying. Jack knew Oz was telling the truth since Jack himself is a expert of lying.
While Jack go to the Intention, Jack told himself that he had no desire to help Oz and fulfill the deal they made. Jack wanted to continue his plan to destroy the Chains. However, Jack found himself questioning his reason for doing all of this. After remembering a memory he had with Lacie shortly after they met, Jack found himself hesitated and instead told the Intention the truth, much to his confusion. To Jack's surprise the Intention forgave Jack for his actions and told Jack she was happy to met him. After the Abyss was restored to its original form after the Intention's death, Jack finally realized his mistake and finally understand that Lacie would never want the worlds she knew to be destroy. Jack thank Oz and Alice for showing him the error of his ways. When Oz told Jack once again how he hated him yet at the same time believed Jack could have chosen a different path, Jack at first was about to agreed with Oz. However, Jack said it isn't possible since he was too mesmerized by the image of Lacie dancing in blood. This sadden Oz.