Tatsuo and Haruko are Shido's adoptive parents. Shido feels greatly indebted to them for taking him in, so much so that he regularly spoils their daughter, Kotori.
In turn, Tatsuo and Haruko greatly trust Shido. So much, in fact, that they left him with the responsibility of taking care of Kotori while they are overseas.
A (seemingly) normal sibling relationship, Shido, and Kotori have been living together in the same household for a long time, and are very close despite the two not being related by blood. Shido deeply cares about Kotori (as a big brother) to the point that, he believed a fake promise made by Kotori which is no matter what happens, even if there's a spacequake, she will wait for him at the family restaurant. Five years prior to the present, Shido gave Kotori a pair of black ribbons as a birthday present. Knowing how much of a crybaby Kotori was, Shido wanted to, in his own way, help his sister grow emotionally, telling her that, so long as she wore the black ribbons, she would become her "strong self".
This would come to somewhat backfire on Shido, as Kotori developed a strong, cynical-like personality while wearing her black ribbons, to go along with her position as commander of Ratatoskr. Despite usually being put off by Kotori's bossy attitude while in "Commander Mode", and her often questionable advice to him during matters concerning the Spirits, Shido still loves his little sister very much.
Mana is Shido's biological younger sister, from whom he was separated from for some years. When they were reunited, Mana immediately hugged Shido out of joy before calling him "big brother", which surprised him. Shido was very shocked to learn that he had a sister and told her that he had no memory of her. However, she said the same goes for her and showed him a pendant that contains a photo of the two of them together as children. Though he had a good time getting used to this event, he was rather accepting of it and welcomed her into his life.
However, there has been lack of major interactions since they've reunited with each other. Nevertheless, it has been seen that he cares very much for her, despite the fact that they both share amnesia towards their childhood memories of each other. It can be seen that the two of them care very deeply for one another and can be by each other's side whenever one of them needs each other. This can be seen on Mana's side for the most part, as she has been helping and protecting her older brother when he went to DEM Industries to save Tohka. She also takes the risk to distract Ellen in order to buy enough time for the Spirits to save Shido in his berserk state.
It is later revealed that Mana was Shido's younger sister from his previous life during the time he met the First Spirit. Back then, the two of them were very close, with both of them being relieved that the other was not harmed by a spacequake attack. Although Mana did share some doubts over the circumstances of him finding the First Spirit, she ultimately chose to trust him after hearing his explanation, showcasing a degree of trust in her older brother. Likewise, their close bond can also be seen when Shido was outraged to hear that Westcott had kidnapped her to exchange for Mio.
Shido shares an especially close bond with Tohka that goes beyond the level of mere friendship. He was the one who gave Tohka her name, based on the day of their first meeting, April 10th. Shido cares very deeply for Tohka, as shown on many occasions, always going out of his way to make her happy and teach her the ways of the world. In fact, his feelings of caring towards Tohka are so strong that, when she was captured by DEM, Shido prioritized saving her above everything else, despite the grave risks involved and the enormity of the odds stacked against him.
Later on, as Shido was about to be killed by Ellen right before Tohka's very eyes, she fell into deep despair and unleashed her Inverse form, going on a rampage that almost cost Shido his life. However, even when faced with Tohka's berserk state, Shido still refused to give up on saving her, ultimately bringing Tohka back to her senses with a kiss; further emphasizing the deep feelings of trust and affection between the two.
When Tohka expressed her dislike of seeing him kiss other spirits after he sealed Yoshino, he tried to keep it through having her avoid seeing him do so to seal other spirits, even though she always ended up walking into the scene each time. Once Tohka learned that Shido had been doing so because it was the only way he could seal Spirits, they both apologized to each other; Tohka for prohibiting the method and Shido for keeping it a secret. Tohka forgave him, but in exchange for his apology of violating their promise without her knowing, she asked him to kiss her equivalent to the number of times he had kept it a secret to make up for that.
Shido considers Origami to be a friend and is the only one capable of reading her intentions and actions despite the doll-like expression she usually has. While normally stunned by her aggressive actions of seductions, Shido takes them all in strides and tries to be good friends with her. Shido has shown a great concern over her vendetta against the Spirits. He even tried to have her to be friends with Tohka and Yoshino, but it eventually turns out to be unsuccessful. When Origami entered her murderous rage thinking that Kotori was the one who killed her parents five years ago, she attacked Kotori during her date with Shido.
After Shido sealed Kotori's powers, he told Origami what he knows about that day, and if she still wanted to kill the Spirit known as <Efreet>, then she can kill him so that she can end the Spirit <Efreet> without killing Kotori. The action was enough to calm Origami down and she passed out shortly afterwards due to overstraining herself by using the <White Licorice>. The Origami of the old timeline has been stated that her feelings of obsession towards Shido were of reliance and not actual love. As of the end of volume 11, the current Origami now states that her current feelings are of love as she tells Shido herself.
Generally, Shido treats Yoshino like a younger sister. After discovering the reason why Yoshino relies so much on her puppet as her idealized role model, Shido promises to become Yoshino's new hero.
The first time Shido met Kurumi, she immediately revealed herself to be a Spirit and attracted his attention. During their first date, he learned she was also the first Spirit he'd ever dealt with who was actively on the offensive. After witnessing Tokisaki kill someone mercilessly, Shido was left horrified and conflicted, unable to think of what to do until Tohka explained to him that in the end any Spirit, like her, could've become like Kurumi if they had been left alone. He thus resolves to save her, despite being terrified of her and knowing that she intends to eat him. Later on, despite his wariness of her, Shido was fully willing to team up with Kurumi in order to save Tohka from DEM.
To date, she is the only Spirit he has failed to seal, however, she has indeed fallen for him, arguably before he even started to date her. She is also very teasing, especially after she witnesses Shido having to dress up as a girl to deal with the Spirit, Miku, and openly jokes about it, much to his dismay.
In Date A Live Encore, a former version of Kurumi meets up with Shido on July 7th for a surprise date. They spend the day together and were a couple until the real Kurumi came to kill her clone from one month prior (the same Kurumi that Shido confronted on Raizen High School's rooftop). Once he read Kurumi's wish she left to hang on a bamboo tree as part of a tradition, Shido climbed high up and tied it onto the tallest tree. He then remarks how that is one wish that needs to come true; and that he'd never forget her.
Shido considers the Yamai sisters as close friends. When asked to choose one of the twins as being worthy of becoming the real Yamai, Shido rejected both options and presented a third choice so that both Kaguya and Yuzuru could live together.
At first, Miku hated Shido, but only because of his gender, as she hates men in general due to an event that happened to her in the past. Upon learning of how she treated people, Shido openly admitted to hating her. Despite this, Shido still cares for her and is willing to help Miku overcome her hate for men, even going as far as cross-dressing in order to just be able to speak to her. Shido also did the dangerous act of stopping her berserk state, and also protected her from several grave assaults. In the end, Miku overcame her trauma and her feelings toward Shido changed. Later on, Miku declared Shido as her boyfriend, calls him "darling", and shows her affection toward Shido without hesitation, much to Shido's chagrin.
Shido had a rough beginning with Natsumi. While their date was originally going smoothly, Natsumi turned hostile when she believed he had discovered her true form, so hostile that she tried to ruin his reputation, erase him and all of his friends in a twisted game. Later, after he saves her from Ellen and tries to convince her that her true form is already cute without her having to hide it, she gradually opens up to him. She has shown some guilt over what she put him through, however, when he remembers being nearly killed by being shot, frozen, eaten, incinerated, blown away or attacked by a huge mob, he considers her actions to be on the level of a mere childish prank. Shido now treats Natsumi as one of his friends and, in a somewhat tsundere manner, she was willing to help him despite her weakened condition when a satellite was dropped on the city.
Shido's relationship with Nia started off relatively well since Nia was grateful to Shido for helping her escape from DEM. However, their relationship momentarily soured when Shido accidentally made her feel like he was mocking her favorite manga character. Nonetheless, Shido was still determined to save Nia. Throughout their interactions, Shido noticed that Nia had a hard time opening up to people since she had a power that would allow her to violate the privacy of anyone who would approach her. In turn, Shido assured Nia that he wouldn't get angry at her if she used her powers to peek into his life. After reading the group's doujin, Nia learned from <Rasiel> the numerous times that Shido had saved other Spirits. She realized that his gentle and kindhearted nature was genuine and could save those who possessed Spirit powers. This realization proved instrumental in developing their bond, allowing Shido to save her by sealing her after Westcott stole the Qlipha Crystal from her.
During their first conversation, Shido was surprised by Mukuro's lack of emotions. She revealed that she used her Angel, Michael, to seal away her emotions. Upon finding this out, Shido tried to convince Mukuro to come down to earth so he could seal her powers. However, Mukuro flat out rejected him, telling him that she is happy with being all alone in outer space now and that his beliefs are hypocritical because coming down to earth would make it easier for DEM to capture her. She then ended their conversation and threatened him that if he appeared in front of her again she would use <Michael> to stop the earth's rotation.
Mukuro rejects him and dismissing his beliefs as a form of hypocrisy had a profound impact on Shido. He began to question if sealing the Spirits' powers may be a mistake, as this makes them weaker and therefore easier for DEM to capture. In the end, Mukuro's words cause Shido to suffer a minor depression, which wasn't cured until he met with his counterpart in the fantasy world.
After having her heart unlocked, Mukuro becomes deeply attached to Shido after seeing his efforts in saving her, and that they were both once lonely orphans that felt empty and were starving for love. Due to becoming attached to Shido, she refused to share him with the others, even locking the memories of all of Shido's friends and family in the process. However, after Shido was able to figure out her past, he empathized with Mukuro's situation due to his own feelings of abandonment during his own childhood. Afterward, Mukuro refers to Shido as her family, and he is the only one whom she allows cutting her hair.
Shido treats Reine like a normal person, although he often tries to correct her when she gets his name wrong.
Westcott seems to know something about Shido's mysterious past, accidentally calling him Takamiya before switching back to Itsuka Shido. Presently, Westcott's plan involves allowing Shido to bond with each Spirit, only to kill him later in order to invert every Spirit that Shido had befriended.
In turn, Shido describes him as someone that can only be called "abnormal" or "bizarre".
He is Shido's best friend. They often talk sometimes in the series.
Friends, acquaintances, and/or classmates, depending on which one he talks to. But overall, he usually has a mundane or normal relationship with any of his classmates. After Tohka and Origami began competing over Shido, however, many of his male classmates became very jealous of him.