Subaru falls in love with her at first glance. He devoted himself to protecting her no matter what happens to him, even if that meant having to repeatedly go back in time by dying. Initially she does not understand the meaning of his playful flirting with her, but gradually she nevertheless develops reciprocal feelings for him and even becomes jealous if she sees his "interest" in other girls. Their relationship is one of the main themes of the series and, in fact, the main reason for Subaru's development as an individual, because thanks to her, he gradually gets rid of his impulsive personality and learns to be more reliable, judicious and useful person than he was at the beginning of the novel. Later Subaru officially becomes her Knight which symbolizes the strong bond of trust she has developed towards him.
After starting out initially as enemies, Rem eventually comes to fall for Subaru after he ends up saving her emotionally and physically during the events oif Arc 2, where afterwards she begins devoting herself to him. After she confesses and wins over a part of his heart after her numeous efforts to support for him in Arc 3, Subaru agrees to take Rem on her offer ploygamy only if Emilia allows it and make her his second wife. In Arc 4 he admits to Emilia that he loves them more or less equally though Emilia remains still in first for him. Subaru was heartbroken to discover that Rem had fallen into a coma and that not even his Return by Death could save her. However, he presses forward knowing that would have been what Rem wanted him to do and in hopes that he can one day free Rem from her slumber.
Subaru and Beatrice initially had an antagonistic relationship. Subaru's personality often annoyed Beatrice, leading her to always mock and insult him. Subaru however begins to rely on Beatrice for knowledge and advice and exclaims how close they are. While Beatrice's actions imply the friendship is one-sided, she does to an extent care for Subaru as she cured him from his curse and foretold him about his death and how he could avoid it. She willingly made the contract to protect him from any potential threats to his life and defended him from Ram's and Roswaal's attacks post Rem's death. After she became his Spirit partner, she starts to open up.
Ram has a rather amicable relationship with Subaru despite barbs she throws at him. During his time as a butler she was in charge of teach him about their daily duties. She had even taken time to educate him on writing so he became more accustomed to life at manor. Ram also the first person to vouch for his character when came to possibility of him being a spy even when his past was still virtually unknown and despite having rather harsh assessment of skills she nevertheless held sense of fondness for him as her junior colleague. Ram would would later come to trust Subaru's intuition even if the likelihood of the scenario had no basis due to his unexpected successes. Although they both hardly mention it, they both care very much for each-others well being and there both good friends
Subaru has a positive friendship with Pack, likely stemming from the boy's friendship with Emilia. However, in a loop where Subaru accidentally killed Emilia by telling her about Return by Death, Pack angrily froze him to death for breaking his promise with Emilia, returning to the mansion against her wishes, and being the one that killed her.
Subaru was originally impressed by Roswaal, admiring him as the head of the Roswaal mansion and Emilia's primary benefactor despite the wizard's eccentric behavior. After the event of Arc 4, he realized that Roswaal is actually an egoistic and cruel person dedicated solely to reviving his master. Among Subaru's allies, he is the first to figure out that Subaru is looping through some method. After Subaru defeated Roswaal in their challenge, Roswaal has agreed to help him, but has also threatened to force Subaru into undergoing another loop if he makes a mistake.
Subaru helped Garfiel to understand the truth about his mother and became good friends with him. Despite his fights in the past, Subaru was still willing to talk to him.
Subaru met Felt in the first loop when some thugs assaulted him but Felt didn't help. Later on, she was about to get killed by an assassin but got saved by Subaru. From there they both established a friendship. Sometime later, when they met again in the capital, Felt showed that she was still making fun of him. Despite the confidence they both have, Felt doesn't like to approach him.
Subaru sees Reinhard as a reliable friend, even bragging to the Witch of Sins that he personally knows the current Sword Saint.
Satella brought Subaru to Lugnica and said she loves him very much. After Subaru met Satella for the first time, she asked him to kill her but he said he will save her instead of killing her.
Subaru has fostered a good alliance with Crusch after the success of killing the Hakugei, with Crusch even giving most of the credit of the victory to him. After her memories got eaten by Ley Batenkaitos their relationship became somewhat neutral. However, after the events in Arc 5 at Priestella, they regained their friendship after Subaru accidentally saved her by taking in the part of the Dragon Vein that Capella used to poisoned her.
Subaru and Anastasia's relationship is somewhat complicated. In one loop where Subaru accidentally told information about Crusch's Camp she abandoned him after revealing that she was actually using him. Despite this, Subaru still saw her as a key potential ally alongside Crusch during the campaign to subjugate the Hakugei. Currently their relationship hasn't improved, because Anastasia is currently possessed by Echidna.
Subaru and Julius started with a bad relation when Subaru forgot his place. While both have apologized to each other after the battle with the Hakugei, Subaru told him he still hated him, with Julius stating that he didn't think they could be friends. After Julius' name got eaten by Roy Alphard, their relationship has gotten somewhat better and became close friends with both liking each other during their trip to the Pleiades Watchtower.
Subaru first thought of Ricardo that he was an annoying guy that would give him a headache. On their way to the Flugel Tree to fight the White Whale, Subaru realized that Ricardo and the members of the Fang of Iron are riding large wolves called Ligers. He asked him a question about a large beast man riding a large wolf makes it a bit awkward. Ricardo didn't mind it, however, he warns him not to ask that question to the others as he considers that question offensive. After they end the conversation on the Lifaus Highway, Subaru realized that Ricardo was looking out for him, calming his nerves before the battle. Subaru's first impression of Ricardo has changed since then. During the fight against the White Whale, he saved Subaru by taking the hit, sustaining major injuries in the process. In exchange, he gives Subaru a message that the whale is "lighter". This gives Subaru a fighting chance.
Echidna is greatly interested in Subaru's Return by Death ability, as it allows him to collect information from multiple points in time. To that end, she invited him to her tea party so in order to strike a conversation with him and later offered Subaru her assistance in the form of a contract. However, Subaru rejected the offer after the other witches gave hints that Echidna wasn't telling the full truth, noticing that she is actually using him.
After the death of Elsa Granhiert, Meli was captured and was overseen under the Roswaal manor. Subaru forgave her and gives her a panda to convinced her to help him from now on.
Even though Subaru doesn't know Ley that well, he feels immense anger for him for placing Rem in a coma and stated that he will avenge her.