Tsukune has a typical relationship with his parents and is a loving son. They were initially worried about Tsukune not being able to get into any high school until the faithful day that his father found a flier for Yōkai Academy. He kept his parents informed of his time at Yokai Academy (albeit without the details of the Yōkai there.) His father is never really shown in depth in the series aside from a few panels in the manga and a short one-liner at the end of the first season anime.
His mother, Kasumi, however had a whole episode/chapter devoted to Tsukune coming home and meeting Moka, Kurumu, Yukari, and Mizore who snuck to Tsukune's house to spend time with him after the forced closure of Yokai Academy. The antics that followed deeply troubled her to the point of calling her husband in a panic to give Tsukune sex-education as soon as possible. After Kurumu, Yukari, and Mizore ran outside to continue their battle outdoors with Tsukune following, Kasumi had a one-on-one talk with Moka, who Tsukune would regularly tell her about. Pleased to know that there is someone there to take care of him, she leaves Tsukune in her hands while he's away in school.
Kyōko is Tsukune's older cousin, though they have a relationship akin to siblings. In their youth, Kyōko often dragged Tsukune into dangerous situations, but the two remained close. Kyōko affectionately calls Tsukune "Tsuki" while Tsukune calls her "Kyō-chan." Kyōko decided to come to Yōkai Academy to see if it is too dangerous during the School Festival. Kyōko got more than she bargained for and saw the true nature of the academy after receiving the Lilith's Mirror. However after Tsukune rescues her from the mirror and chaos that ensued, Kyōko realizes that Tsukune has grown up and doesn't need her to baby him any longer. Kyōko promises to keep Yōkai Academy's nature a secret from Tsukune's parents because she trusts his decision to stay there.
There is hints that Kyōko may have developed some love for Tsukune after he carried her effortlessly in his arms. She noted that he's "so strong" with color in her cheeks.
Over the course of Tsukune's time at Yōkai Academy, Tsukune has managed to garner the affection of a few girls at Yokai Academy. This usually lead to the ire of most of the male student body as many of the girls are among the most beautiful in the entire academy. While his affections are mainly directed toward Moka, Tsukune has a tight relationship with each of the girls in his growing unintentional harem.
Moka Akashiya (Outer)
Moka is the very first student Tsukune met upon arrival at Yōkai Academy. Having fallen in love with her at first sight, Tsukune's thoughts are almost constantly about Moka. Initially he is quite annoyed by Moka's constant habit of biting his neck for blood, he soon realizes that it is a necessity and willingly allows her to do so to keep her healthy. Both hold a mutual attraction to each other, but are too shy to admit their feelings for each other or are interrupted by the other girls when they are about to confess. Nevertheless Tsukune and Moka's bond is strong. After entering Moka's Rosario and seeing her past, Tsukune learns that the Outer Moka is nothing more than an artificial intelligence solely created to protect her Inner self. Despite dropping into depression, Tsukune does not care if the outer one is a fake because he loves her all the same. After Alucard destroys the Rosario seal, mortally wounded, Outer Moka expresses her desire to be with Tsukune forever. The two confessed their love for each other and shared their first kiss before Outer Moka fades. He is willing to do anything for her, even become a true vampire to inject blood into Moka to save her life.
Moka Akashiya (Inner)
Inner Moka and Tsukune's relationship is a little different however. Inner Moka relies on Tsukune to take off the Rosary that seals her, often to fend off students of Yōkai who have attacked either Outer Moka, Tsukune, or both of them. Tsukune is initially intimidated by her Inner form but he still finds her extremely beautiful nonetheless. Though Inner Moka is initially cold to Tsukune, she warms up a little to him and eventually falls in love with him herself, but is too stubborn to admit it. When Tsukune is about to die after a brutal attack from ANTI-THESIS, Inner Moka injects him with vampire blood despite the consequence of turning him into a ghoul, saving his life at the cost of his humanity.
In their second-year, Tsukune begins training with Inner Moka and Inner Moka's developing affections for Tsukune becomes more apparent. When Inner Moka and Tsukune reunite at Fairy Tale Headquarters for the final battle, she opens her heart to him.
Tsukune first met Kurumu after hearing rumors of a girl who was seducing the male student body. Kurumu herself made advances on Tsukune after hearing that her main rival, Moka, who had stolen nearly all of the attention of the male students, was close to Tsukune. Though Kurumu had Tsukune under her Allure, Tsukune could not shake his thoughts of Moka. Angered, Kurumu tried to attack Tsukune only for Moka to step in and save him. Just as Inner Moka is about to mangle Kurumu, Tsukune stops her and tells her that he does not think that Kurumu is a bad person. Touched by his kindness, Kurumu develops true love and affection for Tsukune.
Tsukune and Kurumu's relationship afterwards is a completely friendly one, though Kurumu often hugs Tsukune using her chest, often suffocating him. Because of Tsukune's love for Moka, Kurumu's affection for him goes generally unrequited. In their second-year after Moka is kidnapped by Fairy Tale, Kurumu reveals that Succubi live off of love, and with her generally one-sided love for Tsukune, her health drops. Shortly after, Tsukune loses control of his Shinso-Ghoul blood and goes berserk. Kurumu arrives at the site where Touhou Fuhai has been injured. Separated by a barrier, Touhou reluctantly decides to kill Tsukune. Horrified, Kurumu tries to break through the barrier, despite taking heavy damage. Fueled by her true love for Tsukune, she gives Tsukune his first kiss, stopping him long enough for Touhou to throw the final needle to seal Tsukune's power. Afterwards Tsukune and Kurumu share a tender but somewhat shy moment before heading off to rescue Moka.
When Yukari was first introduced, she was only concerned about Moka, wanting to have her all to herself. She saw Tsukune as an obstacle standing in the way of her goals. Despite this, Tsukune held no ill will towards her, despite her constant pranks on him during this time. Tsukune helped save Yukari from prejudice monsters, accidentally gaining her affections in return. However, this lead to an unwanted change in her goals as now, Yukari wants to have a three-way relationship Moka and Tsukune. As time went on. Tsukune began growing more and more frightened of Yukari due to her obsession to create a three-way relationship between them; however, he respects her knowledge and magic, which has helped them out of tight spots more than once.
Originally, Tsukune was targeted by Mizore because she thought he was lonely as her. At the very least, Mizore was interested in Tsukune, but after he saved her from Okuto Kotsubo, Mizore fell in love with him. Mizore's stalker tendencies grew after she became a part of the group and she often throws ice kunais at any of the girls who get too close to Tsukune. Tsukune is often at unease while around her, especially when she brings up her desire to "make babies" with him, but he generally appreciates having her around.
Before the final battle against Fairy Tale, Mizore reveals that she does not care who Tsukune chooses in the end as she will always love Tsukune regardless.
Ruby is first met during the Newspaper Club's outing to the human world. Upon arriving at a Sunflower Patch known as the Witch's Knoll. There they meet Ruby who attacks them because humans were planning on demolishing the Witch's Knoll. After knocking her out, Tsukune and the rest of the club take her to an inn and Tsukune apologizes for the actions the humans have done and Ruby begins to believe that humans and yōkai can coexist. After battling her master Lady Oyakata, Ruby spends some time recovering from the ordeal.
After Tsukune begins his descent into a ghoul, the Headmaster arrives with none other than Ruby to seal Tsukune's transformation. Ruby has revealed that she has developed feelings for Tsukune during her absence and works at Yōkai Academy to be close to him. Her relationship with him differs between the Manga and Anime. In the Anime, she tends to have a desire to be the more dominant personality over Tsukune while in the Manga, she tends to be more submissive, wanting him to be her "master." While Tsukune is disturbed by her sexual play advances, nevertheless, he likes Ruby as a friend.
Friend. Tsukune was originally at odds with Gin because of him wanting to date Moka. However, this problem dies out after the fight with the Security Committee. Tsukune respects Gin's noble and honorable side, but dislikes how much of a pervert his club president is.
Moka's younger half-sister. Kokoa is extremely protective and possessive of her older sister, Moka when she is in her monster form. She initially saw Tsukune as a baggage on her sister and tried her best to sever their ties to each other. However the two developed something akin to friendship during Kokoa's first few weeks at Yōkai Academy. Kokoa's hostility to Tsukune has lessened over time and she eventually works with Tsukune along with the rest of the Newspaper Club, which she joins.
Kokoa is notable as the only female character so far that has not shown any romantic affection for Tsukune.
Upon introduction, Fang-Fang desired to have Tsukune join his clan, the Huang Family. Unfortunately his recruitment speech was misinterpreted by everyone as a marriage proposal. For much of his initial chapters, Tsukune saw Fang-Fang as a nuisance and, because of his awkward introduction, uncomfortable to be around. However, with time, Tsukune began to see Fang-Fang as a more reliable individual, but still does not want to join his clan.
Hokuto was first seen by Tsukune as a strong and trustworthy individual. However, upon the revelation that he was masterminding ANTI-THESIS, Tsukune was crushed. During their battle, Hokuto revealed that he was once a human himself until he had monster blood injected into him like Tsukune. During their battle, Tsukune helped Hokuto see past his hatred of monsters and find the kindness in his own heart. After the battle, Hokuto went into hiding. Deep down, Tsukune still wanted to be his friend. During the Fairy Tale arcs, Hokuto became a valuable ally to Tsukune and his friends and helped him infiltrate Fairy Tale's headquarters and even offered his life to assist Tsukune in anyway possible to reach Moka.