Ritsuka Tachibana
Rem appears to be using her for his plans in obtaining the grimoire. But in the recent episodes, he seems to care for her and has started to develop feelings. He is visibly moved when she runs to him in the second episode, relieved that he is unhurt. He actually calls her by her name for the first time in the third episode, after he saves her from Urie, and attempts to go after her when she runs from them. Urie then asks him, if everyone is simply his pawns, why does Ritsuka have him "so hot under the collar." In the fourth episode he tells himself that he must find the grimoire, but then has a moment of doubt while thinking of Ritsuka. Even going so far as question himself on what is important to him now. In the fifth episode he reluctantly tells her that he is just using her as a "pawn," yet, despite his cold words, when she backs away from him he reaches out to her and appears hurt when she tells him to stay away from her. After Mage takes her through a portal, he angrily says that she is his and no one else's.
In episode 7, Rem is seen overhearing Ritsuka's and Roen's conversation about how she is the forbidden grimoire with a troubled look on his face. He then stops Ritsuka from taking off her protective ring, thus defying his father and losing the quickest way to obtain the grimoire. When Roen takes her hostage and threatens to kill her, Rem angrily tells him to let her go. When Lindo arrives and flies into a blind rage, he protects her. At one point, he could've dodged Lindo's attack, but shielded Ritsuka instead, getting injured in the process. When he feels Lindo gaining the upper hand, he desperately asks him if he would kill Ritsuka, too. This brings Lindo back to his senses and Rem departs. He is later seen at his home declaring checkmate, yet he seems to have second thoughts about it; his eyes shimmering with emotion. He is shown to have feelings for her in their song in episode 8.
In episode 9, Rem is seen throughout the episode looking for Ritsuka, finally having decided that he loves her more than he wants the grimoire. Later, he catches Lindo about to kiss Ritsuka and appears shocked. When the two sing of their love for her, Rem declares that he will go against his previous ideals, firmly declaring that Ritsuka is his new destiny. He appears shocked when she yells at him for hurting her brother, and looks hurt when she states that all he cares about is the grimoire and nothing else. She then tells him that she hates him and never wants to see him again; he appears heartbroken at this. It is then that Rem realizes he waited too long to tell Ritsuka the truth of how he feels, and sadly leaves with a broken heart, regretting everything he'd done. However, his feelings for her do not go away.
In episode 10, Rem is shown to be a little heart-sore. However, he gets extremely jealous when Urie, Mage, and Shiki confess that they love Ritsuka. This prompts the three of them to demand that Rem confess the truth of his feelings for Ritsuka, which he does not. At the end, he is shown looking at the crown from the school ball, and whispering her name. In episode 11, with some encouragement from Roen, he finally admits and accepts his feelings for her, going with the other five to save her from the vampires. While there he admits to her that he loves her.
In episode 12, Rem shielding Ritsuak from Nesta's final attack serves as a catalyst for Ritsuka to activate the grimoire's powers and kills Nesta once and for all. Later, by the lake the two confess their love for each other. Rem asks Ritsuka, being part-devil to come and live with him in the devil realm, but Ritsuka refuses because of Lindo and her mother. The two hug and share a kiss before Rem disappears in front of her eyes, but not before promising to meet her again some day. In the aftermath, Ritsuka wears Rem's king crown as a necklace, as a token of their time together and their love for each other. In the final song Ritsuka declares that she'll love Rem "forever and ever."
It can be presumed that Rem keeps Ritsuka's queen crown. Also, the promise to reunite with Ritsuka heavily hints that their love story is not finished.
Lindo Tachibana
Rem and Lindo have hated each other since the moment they met, and those feelings of hatred have only increased with time. Once they both realize that their feelings for Ritsuka are the same—though Lindo believes that Rem is only pretending in order to obtain the grimoire—their relationship only worsens, turning into a fiery rivalry.
Rem catches Lindo, when he is about to kiss Ritsuka in episode 9, and appears shocked by it. At a lost for words, and struggling with his own feelings, Rem agrees to settle his differences with Lindo right then and there. A violent fight ensues between them for Ritsuka's heart, ending with Rem almost killing Lindo, before he is stopped by Ritsuka. In episode 11, they team up to save her, signifying a begrudging respect.
Lindo was surprised about Rem's genuine feelings for Rituska but accepts the fact that the two love each other. He even shows his maturity by being ready to let Ritsuka go with Rem if she chooses and protect her from afar.
Urie Sogami
Rem and Urie have been friends since childhood. As of now they have a respectful, albeit cold relationship. With Rem getting easily annoyed whenever Urie goes against him. In episode 8, he appears shocked and jealous when Rem and Ritsuka dance together, hinting that he can tell Rem has fallen in love with her. This is confirmed in episode 10, when Urie bluntly tells Rem to confess that he loves her too. When he refuses, Urie says that until he confesses the truth, none will trust him to keep her safe. When Rem finally confesses his true feelings in episode 11, Urie was shown to be very accepting and satisfide, saying that he like his childhood friend better when he was "straightforward...and vulnerable." In episode 12, he was surprised by Rem's open display of love for Ritsuka, and while he was sad by their mutual confess, he was willing to let it happen since it was real. However, beofre he leaves he promises to return some day for Ritsuka, because his love, like Rem's, is eternal.
Mage Nanashiro
Rem thinks of Mage as one of his pawns and is shown to be annoyed with Mage whenever he, like Urie, goes against him. Whenever Mage abducted Ritsuka, he verbally berated him for ruining everything he'd so carefully put together with her. Mage has outright said that he is tired of being one of Rem's chess pieces, and tells him to go "play his little games by himself."
In episode 8, he gets jealous when Rem dances with Ritsuka. In episode 10, he, along with Urie and Shiki, demand that Rem tell them the truth about his feelings for Ritsuka. When Rem refuses, Mage, along with Shiki, is shown agreeing with Urie that until the danger has passed they can't trust him with Ritsuka. He finally trust him again in episode 11, when he confesses his love for Ritsuka. In episode 12, while he was sad at Ritsuka and Rem's mutual love confession, he was willing to let it happen since it was obviously real. However, before he leaves he swears to return and make Ritsuka his some day, indicating that his love for her, like Rem's, is eternal.
Shiki Natsumezaka
Rem and Shiki don't have much interaction, but Rem, like the others, seems to hold nothing but contempt for Shiki, presumably because he is a fallen angel. He gets jealous of Rem in episode 8, when he gets to dance with Ritsuka. In episode 10, Shiki says that he will allow the Arlond family to have the grimoire, but he wants Ritsuka. This provokes a jealous Rem, Mage, and Urie to snap at him. He is then shown telling Rem to confess his true feelings for her, and when he doesn't Shiki agrees with Mage and Urie to not trust him with Ritsuka. Shiki was satisfied with Rem's confession in episode 11. In episode 12, Shiki expresses his sorrow at Ritsuka and Rem's mutual love confession, but was willing to stand aside since it was genuine. However, in the end, before he leaves, he promises to return some day and fall with Ritsuka on that day, signifying that his love for her, like Rem's, will remain eternal.
Roen
Roen was Rem's pet dog, but their relationship is shown to be rather stiff. Rem allowed Roen to follow him to and from school without really acknowledging him. However, in episode 7, when Roen attempts to put Ritsuka into a deep slumber Rem stops him. When confronted, the former dog says that his loyalty was to Lord Maksis and no other. Rem becomes furious, yelling that he should know better than to try and order him around. When Roen threatens Ritsuka, Rem demands that he let her go.
Later, in episode 11, he helps Rem to come to grips with his feelings at long last.
Mikukier Arlonde
Rem is shown to have a very abusive relationship with his father, one that is more like master and servant. His father is shown to feel no love for his son, which could explain Rem's icy personality. This might also be why Rem falls in love with Ritsuka because she is the first person to genuinely care for and be kind to him.
In episode 9, Rem decides to go against his father's plan—no small declaration, considering how frightened and nervous Rem was when in his presence—and make Ritsuka his new destiny. This indicates that Rem loves Ritsuka, more than he fears his father.