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 and heartbroken when she yells at him for hurting her brother, and looks hurt when she states that all he cares about is the grimoire, nothing else. She then tells him that she hates him and never wants to see him again. 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.
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.
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.
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.
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.
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.
Unknown Father
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.