Yuu encountered Iris for the first time on a beach upon arriving on Midgard, although their meeting was awkward as her swimsuit had washed away, leaving her naked in front of him, which led to Iris mistaking him for a pervert. Soon, however, the two started to get along as classmates. After Yuu saved her from Leviathan, Iris expressed her gratitude by kissing him. From that point on, Yuu started developing feelings for her. At the end of Volume 3, Yuu confesses his feelings to Iris, but then tells her about his deal with Yggdrasil and his memory loss, along with how Mitsuki might be the one he truly loves, leaving their relationship stagnant. In Volume 4, Yuu tells Iris about what little memories he can recall in the event that he should forget them, showcasing his deep trust in her. Even after regaining his memories, Yuu's feelings for Iris remained strong, and his desire to not lose her led him to instinctively mark her as his mate.
She is Yuu's childhood friend and non-blood related sister. When they were young, Yuu promised to marry her when they grew up and she kissed him on the lips as a sign of their engagement. However, as a result of the deal he made with Yggdrasil in order to defeat Leviathan, Yuu forgot about their promise and started treating her as his actual sister. He was made aware of his forgotten promise by Mitsuki, something which put a damper on his budding relationship with Iris Freyja, since Yuu was unsure whether he loved Iris or Mitsuki more. Even after regaining his memories, his uncertainty lingered. Regardless, it's obvious that Yuu treasures Mitsuki very much, evident by the fact that he almost killed Vritra for poisoning Mitsuki. In Volume 11, Yuu clearly stated that Mitsuki is the one dearest to him, but was told by Vritra that his intention to stay close to her was all because of his instinct due to the Ninth True Dragon inside Mitsuki, possibly rendering his feelings for her fake.
Originally, their relationship started rather cold, as Lisa was against Yuu joining Midgard, although Yuu was determined to gain her approval. Lisa and Yuu's relationship improved after he contributed to Leviathan's defeat and helped her mend her relationship with Mitsuki. The two of them grew closer during the couple training they did in Volume 5, in order to convince Lisa's parents that the two were boyfriend and girlfriend. It was also then that Yuu revealed his deal with Yggdrasil to Lisa, showing how much he trusts her and her judgement. He also gained a better understanding of her during this event, allowing him to easily discern her thoughts. When Yuu marked her as a mate, he promised to take responsibility after Lisa's outburst.
Yuu met Tear for the first time during one of NIFL's operations, where he saved her from a criminal group. Even though he found out that she was a D, Yuu opted to let her be with her parents instead of taking her into custody. They were reunited in Midgard, when Tear was brought as a new student and proclaimed Yuu to be her husband. Initially, Tear's extreme attachment to Yuu and her firm belief that she was a Dragon caused him a lot of inconvenience, but he was able to get her to open up and change her views with the help of his classmates. Yuu has been shown to care very much for Tear, but becomes awkward whenever Tear proclaims her love for him and attempts to get closer to him, mostly due to the fact that she is very young and his own complicated feelings for Iris and Mitsuki. However, this didn't stop him from marking Tear as a mate, further showcasing his care for Tear.
At first, Yuu didn't socialize much with Firill due to the latter's quiet personality. However, he still considers her a good friend, evident by how he rushed to comfort her after the first battle with Hraesvelgr and the effort he put in order to destroy the Dragon and save Firill. After Firill developed feelings for him, Yuu would get instantly embarrassed whenever she showcased her affections for him. He cared enough for her to mark her as his mate in Volume 8 and was shown feeling slight regret that their marriage was null outside of the Principality of Erlia in Volume 12, implying that he might also have feelings for her.
Yuu and Ren had a very distant relationship at first due to Ren's reluctance to speak to other people. During the events of Volume 6, the two of them grew closer after Yuu learned the circumstances of Ren's past and confided in her the deal he had made with Yggdrasil. This led to Ren starting to call him "Onii-chan", something which Yuu consented to and which led to him starting to treat Ren as his younger sister as well.
Yuu had a casual relationship with Ariella, typical of that between two classmates. However, he did care enough for her to mark her as a mate during the incident with Kraken Zwei. When Ariella was revealed to be a spy from NIFL, Yuu paid no heed to it and asked Ariella to come with him, stating that he would hold her and never let go, showing that he does indeed have some feelings for her. Yuu has stated that he finds her legs to be her most attractive feature, even finding them distracting during battle.
Yuu met Kili for the first time three years before the start of the series. However, due to his deal with Yggdrasil, he forgot about that encounter. When they met again, Yuu was hostile towards her because of her notorious reputation and the fact that she attempted to abduct Tear. He came close to killing her during their fight, but stopped after recalling the time they had spent together when Kili was disguised as Honoka Tachikawa. In their subsequent encounters, however, Yuu's hostility towards Kili lessened considerably after he learned the details of her past and recalled the memory of their first encounter. This led to Yuu becoming more aware of Kili as a girl, stating once that he found her very attractive. During the incident with Kraken Zwei, Yuu promised that he would definitely protect Kili this time, showing that he has grown to care for her. In Volume 8, Kili was also among the Ds Yuu marked as his mates, further showcasing his care for her.
Yuu encountered Jeanne for the first time when he saved her from an ambush, but was unaware that she actually cross-dressed as a man named John in order to fit in at NIFL. The two became friends during their time at Sleipnir, but Jeanne did not reveal her secret, fearing that Yuu would reject her. However, Yuu eventually found out that she was a girl in Volume 8 after witnessing her bathing, much to his surprise. He proceeded then to comfort Jeanne, stating that her gender didn't matter to him and that he still trusted her, prompting Jeanne to burst into tears and declare that she would follow him for the rest of her life.
Originally, Yuu viewed her as an out of control enemy and was determined to kill her for seemingly killing Haruka and in order to protect his friends. During their fight, Yuu unintentionally marked her as his mate, which transformed her into a human and changed her personality into that of a young girl. Now, Yuu no longer holds a grudge against her and even treats her like a daughter due to Shion calling him 'Papa'.
Yuu and Charlotte have a friendly relationship, although Yuu gets exasperated and embarrassed whenever Charlotte tries to have him join her in peeking at girls changing or whenever she creates misunderstandings that the two of them are in an intimate relationship. However, he is also very grateful to her for the aid she provided to him during the Yggdrasil incident. Their friendship was further cemented during the events of Volume 10, where Yuu fought against NIFL in order to protect her. He even went as far as to call her his best friend during Brynhildr Class' barbecue party, showing that he has grown to value her greatly.
Haruka is Yuu's homeroom teacher in Brynhildr Class, whom he has a good amount of respect towards to. Yuu also thinks of her as an older sister, being extremely horrified upon seeing her being impaled by Kraken Zwei, and was relieved to the point of crying upon learning that she was still alive.
Yuu's superior officer during his days as a NIFL soldier. He trained Yuu to be his most prominent assassin and has great trust in his abilities. Because of Loki's personality and manipulative nature, Yuu greatly dislikes him and mistrusts his methods of doing things. However, after Midgard's invasion, their relationship appears to have become more cordial.
Yuu was forced to make a contract with 'Green' Yggdrasil three years before the start of the series in order to destroy Hekatonkheir, sacrificing a part of his memories to the Dragon for compensation. Despite that, Yuu trusted Yggdrasil and called upon his help several times in order to overcome the dragons. However, Iris and Lisa's words made Yuu starting to doubt Yggdrasil's motives. His fears were confirmed when it was revealed that Yggdrasil had also downloaded something akin to a computer virus to his brain with the intent of taking over his body. Thanks to Charlotte and the Dragon Subjugation Squad, Yggdrasil was neutralized and Yuu was freed from his influence.
A Dragon who started out as an antagonist towards Yuu and his friends. While he tried to get along with her at first, Yuu almost killed Vritra after believing that she had killed Mitsuki following Brynhildr Class' confrontation with Kraken Zwei. Afterwards, Yuu and Vritra's relationship became rather cold, but when Yuu was revealed to be Neun, the two grew closer due to Vritra's desire to be establish a friendly relationship with him, even if it meant having intercourse with him.
Yuu's parents that he hadn't seen or spoken to for three years after being taken by NIFL. Following the loss of his memories during the battle against Hekatonkheir, Yuu forgot everything about his parents and was reluctant to meet or speak to them, fearful that this would reveal his memory loss to them and Mitsuki. Upon meeting them after so long, while feeling nervous around them at first, Yuu devoted himself to remember what he could about them. After Yuu regained his memories in Volume 7, it is likely that he's now able to communicate with his parents normally.
Yuu grew to resent Kenya upon learning how neglectful he was towards his daughter Ren. In spite of that, Yuu attempted to rebuild their relationship, an attempt which however failed and led to Kenya rejecting Ren completely, causing Yuu to further hate him. However, Yuu suspects that Kenya still cares for Ren and hopes she will find happiness with him instead.