Ritsuka Tachibana
They have a strange relationship, with Shiki seeming to enjoy teasing Ritsuka, and taking pleasure from her difficult situation. Shiki claims that she and him are the same. When he traps her in the illusion of a museum he tells Ritsuka to stay with him and be his. If not, he'd kill her. He then claims that he won't let a liar touch her whenever it is revealed that Azuna is an exorcist. He seems shocked whenever she stops him from killing Azuna, and seems to be genuinely moved when she claims that a lie told with good intentions is not a sin. He then says that she's no fun, and returns them to the fountain. He also seems surprised by her kinder personality, since he's often exposed to the more abrasive Urie and Mage. He claims that lies are truth and that she should decide for herself whether or not he was lying about her mother.
He asks her to go to the dance with him in episode 8, and gets jealous when she dances with Rem when they are chosen as King and Queen. In episode 9, he is shown throughout the episode looking for Ritsuka once he learns that she is the grimoire. He fights the exorcists, along with Mage and Shiki, when they threaten to kill Ritsuka. In episode 10, he asks that she be given to him. He says that he knows the grimoire belongs to the Arlond family, and that he's fine with that, but he wants Ritsuka. This causes him to be threatened by a jealous Rem, Urie, and Mage. He also agrees when Urie says that until Rem can admit his true feelings they can't trust him with Ritsuka.
In episode 11, he desperately searches for her with the others, and is shocked to find that she is at the vampire castle. He later goes with the others to save her, and confesses that he loves her, saying that when he thinks of her "his chest stings and tingles." She is touched. In the final episode, he fights to protect her, and while he is saddened by her mutual confession to Rem, he is accepting of it. However, before he leaves, he vows to return, expressing his hope that he and Ritsuka can one day fall together. This indicates that his love for her remains eternally unchanged, and that she will forever be the only girl he desires.
Mage Nanashiro
Shiki seems to enjoy teasing Mage. Popping out of nowhere in episode 5, when Mage had been reminiscing over the time he'd spent with Ritsuka, and startling him. He then proceeded to mock him about how red his face was when he was thinking of her. Mage then angrily denied this, telling Shiki that if he brought it up again he'd murder him. Shiki playfully brushed this off, annoying Mage even more. He later makes Mage jealous in episode 10, when he says that he wants Ritsuka. However, the two agree that until Rem can stop lying about his feelings they can't let him near Ritsuka. They team up in episode 11 to save Ritsuka.
Urie Sogami
Shiki and Urie have an abussive, albeit respectful, friendship of sorts. With Urie often threatening to strike Shiki—and actually slapping him with his rose once—with his "thorny whip." This often makes Shiki excited due to his sadomasochistic nature. He causes Urie to get jealous when he declares that he wants Ritsuka in episode 10. However, he does agree with Urie when he says that they can't trust Rem with Ritsuka as long as he denies his feelings. They later team up with the others to save Ritsuka from the vampires.
Rem Kaginuki
Shiki respects Rem enough to allow him to lead. However, he had no qualms about going against him. In episode 10, when he says that Rem can have the grimoire, but he wants Ritsuka, which makes Rem jealous. They later go to save Ritsuka from the vampires.