The two of them are cousins but were raised as siblings. Ritsuka refers to him as her prince since he's always protected her since they were young. Unbeknownst to her, Lindo has been in love with her since they were children and would do anything to protect her; even if it means his life.
In episode 9, she is very accepting of Lindo's nature as a Dhampir, but is shocked when he reveals that he is in love with her. In episode 11, when he and the devils go to save her, Ritsuka is shown to be touched by their declarations of love. In episode 12, he kills Jek and attempts to kill Nesta for harming Ritsuka. Once both are dead, and the grimoire vanished to places unknown, Lindo was shown watching Rem and Ritsuka confess their love for each other, sad, but accepting, and promising that he would continue to show his love for her by always protecting her.
Maria is Ritsuka's mother. She and Ritsuka get along well, and it is apparent that the both of them care for each other.
Maria is shown to feel guilty about Ritsuka's devil lineage. However, Ritsuka doesn't blame her, and forgives her for keeping it hidden form her all those years.
The two of them are best friends. It is apparent that Azuna cares for her and seems to know about the whole devil and grimoire situation. This is confirmed in episode 6, whenever it is revealed that she is an exorcist like Ritsuka's cousin, Lindo. She is shown to regret deceiving Ritsuka and attempts to protect her from Shiki. She later dies in Ritsuka's arms in episode 8, still considering her a dear friend.
Lindo later reveals in episode 9 that Azuna was afraid of Ritsuka finding out that she was an exorcist, because she feared that they would no longer be friends. However, Ritsuka accepted it instead, and she was overjoyed that they could still be best friend and she could openly protect her.
At first, Ritsuka didn't really think much of Rem, calling him a jerk after their first encounter. However, she soon changed her mind when he saved her from Jek and his Devil Worshippers, and was visibly touched when he promised to help her find her mother. She is also shown to be concerned for him when her grandfather's house is set on fire, rushing to him in relief when he emerges unharmed. She came to think that he was helping her out of kindness, and she refused to believe that he was a devil when Lindo told her (Possibly starting to develop feelings for him). She trusted Rem, and felt utterly betrayed when, in episode 5, he said he was just using her as an "obedient pawn." Later, after Mage has snatched her back, she cries on the beach, feeling hurt, used, and betrayed. She was hurt, not because Rem confirmed that he was a devil, but because he told her that she was just his pawn. Though whether he actually meant those cruel words is another matter entirely. Later, in episode 7, he stops Ritsuka from taking off her protective ring, and putting herself into a deep slumber.
With this act, he goes against his father and stops what would've guaranteed him the grimoire. When asked why, he firmly states that he won't let her go. Once Lindo shows up and flies into a rage, Rem protects Ritsuka from him, putting his own life on the line in and getting injured in the process. Afterwards, he is seen proclaiming checkmate on his board, with a piece that supposedly represents Ritsuka, yet he seems to have second thoughts about it; his eyes are shown glistening with emotion. She is shown to have feelings for him in their song in episode 8.
However in episode 9, Ritsuka was hurt that Rem is willing to hurt the people she loves in order to get the grimoire. She tells him she hates him and she doesn't want to see him again. In the end after taking her brother to school, she decides to forget her feeling for Rem in order to protect her family. She later regrets this choice in episode 10, thinking about Rem with tears in her eyes. She is shown to be touched when he came to rescue her, and by his declaration of love.
In episode 12, when Rem gets injured while protecting her from Nesta's final attack, Ritsuka activates the grimoire's powers to protect Lindo, Rem, and the devils and successfully stops Nesta once and for all. After the battle and the disappearance of the grimoire, 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. At this, Rem promises to meet her again one day, and the two hug and share a kiss before Rem disappears. In the aftermath, Ritsuka wears Rem's king crown as a necklace- a reminder of their time together and their deep love for each other.
It is presumed that Rem has Ritsuka's queen crown with him. Also, the promise to reunite with Ritsuka heavily hints that their love story is not finished.
Ritsuka is shown to be greatly immune to Urie's charms and doesn't fall for him like other girls. In fact, the only way he can even get close is by hypnotizing her; he gets rather annoyed when he can't have his way with her. He seems to have a genuine interest in Ritsuka, calling her his special butterfly.
In episode 8, he is hinted to have slight feelings for her when his interest in her only increases after he believes that the grimoire has never been with her or her family. He asks her to be his partner at the dance and appears to be jealous and surprised when she and Rem dance together.
In episode 9, when he learns that Ritsuka was the forbidden grimoire all along, he begins searching for her. He fights the exorcists, along with Mage and Shiki, whenever they threaten to kill her. In episode 10, Urie gets jealous whenever Shiki bluntly says that Rem can have the grimoire, but he wants Ritsuka. Urie claims that he won't let anyone have her, because he loves her too. He also calls out Rem for hiding his feelings, saying that as long as he insists on telling lies, then none of them can trust him with Ritsuka. This indicates that Urie is now long past caring about the grimoire, and instead now only seeks to keep Ritsuka safe until the dangerous time has passed. Also, during the song "Emo Liar," Urie calls Ritsuka "the sweet little light that brightened up your gloomy days." This could be an indication of how he sees her.
This is proven correct in episode 11, when Urie is shown to be desperately looking for Ritsuka, deciding to take the fight to the vampire king himself to save her. He later confesses to her, along with the other five, that he's fallen in love with her. She is touched. In episode 12, he fights to protect Ritsuka, and in the aftermath is shown watching her and Rem together. He appears sad but pleased, and leaves, promising to return one day for Ritsuka, but asking her to take care in the mean time. This shows that even after everything, even after learning the truth about Ritsuka, Urie is still in love with her, and like the others, always will be.
Like with Urie, Ritsuka is immune to Mage's charms, causing him to get extremely frustrated when she didn't fall for him like most souls did. She is shown to be annoyed at him when he abducts her, and tried to escape every chance she got. After Rem revealed that he was using her, Mage took her to watch a sunset to cheer her up. Hurt by Rem's betrayal, she allowed herself to cry in front on him. She did seem to genuinely feel something when they stared into each other's eyes, and was concerned when Mage and Lindo fought, casting herself between them before they could kill each other.
The suspicion that he has feelings for her is further confirmed in episode 8 when he believes that she knows nothing of the grimoire and never did his interest in her doesn't waver; if anything it strengthens. When he asks her to be his partner at the dance, he blushes and says that while dances aren't his thing, if she wanted to dance then he wouldn't mind. He is also shown to be jealous whenever Rem and Ritsuka are chosen as King and Queen and dance together.
In episode 9, he begins searching for her when he learns that she is the grimoire. He tries tracking her using his nose, which leads him to the Exorcist Society, where Urie and Shiki are already waiting. He fights the exorcists when they threaten to kill Ritsuka. In episode 10, he gets jealous when Shiki says he doesn't care for the grimoire, and asks for her instead. Mage says that she is his, that he wants her laugh and teary face—all of her. He also gets jealous when Urie enters the argument, saying the same. When Rem continues to deny his feelings for Ritsuka, Mage angrily tells him to admit that "he's not after her—he wants her!" When he still refuses, Mage is shown agreeing with Urie that they can't trust him not to hurt Ritsuka. This shows that Mage no longer cares for the grimoire, but Ritsuka's safety instead. During the song "Emo Liar," Mage says that all of their feelings were "born of sensing that warmth." This is valid, seeing as how he began to fall for her from the moment he looked into her eyes on the beach.
Later, in episode 11, he is shown desperately looking for her, and agrees to go with the others to save her when he learns that she is at the vampire castle. He later confesses his love for her, saying that it is real. She is touched. In episode 12, he fights the vampires to save her, and in the aftermath watches her with Rem by the lake. He appears sad but accepting, and leaves promising Ritsuka that he would return one day, and that she would be his. This indicates, that even in spite of everything that has happened and everything that she is, Mage still loves Rituska, and always will.
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. Whenever she gets pricked by a thorn he stares at her intently. Also, when he traps her 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. Before leaving, he claims that lies are truth and that she should decide for herself whether or not he was lying about her mother.
In episode 8, Shiki asks her to be his partner at the dance, and appears slightly jealous and worried whenever she and Rem dance together. In episode 9, he is surprised to learn that she is the grimoire, and spends the episode searching for her. He fights exorcists, along with Urie and Mage, when they threaten to kill Ritsuka. In episode 10, he tells Rem that he's okay with the Arlond family having the grimoire, but he wants her. He gets threatened by a serverly jealous Rem, Mage, and Urie as a result. When everyone starts confronting Rem about his feelings for Ritsuka, Shiki reminds him more than once that she isn't just the grimoire, she's a human girl. Shiki is then shown agreeing with Urie and Mage when they say that as long as Rem insists on lying, they can't trust him not to hurt Ritsuka. When they sing "Emo Liar," Shiki says "the world turns with that gentle touch." This could be an indication of how he has come to see her.
In episode 11, he is shown to be desperately looking for her. When he learns that she is at the vampire castle, he agrees to go with the others to save her. He later confesses that he loves her so much his "chest stings and tingles." She is touched. In episode 12, he fights the vampires to protect Ritsuka, and is shocked when she kills the vampire king. In the end, he was shown to be sad but content with Rem and Ritsuka confessing to each other, and leaves promising that some day he would return, and on that day he and Ritsuka would fall together. This indicates that even after everything, after learning the truth about Ritsuka, Shiki is still in love with her, and always will be.
Maksis, known as the king of devils who was defeated by Rem's father, is Ritsuka's father. The father-daughter relationship is currently unknown, though he knows of her and was apparently overjoyed at her birth, though it was only for the grimoire. Ritsuka, based on intel from Jek, knows about her father but seems to be aware that her father does not love her just like how her uncle does not love Lindo.
However, it is revealed in the game that Maksis truly did love Ritsuka and sent Roen to protect her before Mikukier Arlond sealed him away, which Roen severely misinterpreted.