He's protective of her. She is Kojou's sister and the classmate of Yukina.
A 14 year old middle-school Sword Shaman from the Lion-King organization, Yukina was sent to observe the fourth primogenitor and was told to kill him if he becomes a threat. At first, Kojou tries to trick her into thinking he's the wrong person, but his strength is revealed when he intervenes in a fight between her and another vampire. She is constantly following him everywhere he goes being as his observer and as a result has become very close to him, even allowing him to suck her blood in order for him to gain control over his familiars. Unbeknownst to her, she was actually sent for this very purpose; with the organization intending for her to be his first blood partner, and even his mate. Like many others, she was supposedly sold to the Lion-King organization by her parents.By the end of the first arc, she seems to have developed feelings for Koujo whilst Kojou had shown deeper feelings for her when he realized 'something' after Sayaka reprimanded him for letting Yukina walk away from his life. He runs after her to stop her from leaving but was held back when an attack occurred at the island. In Strike the Blood EX, she is presumed to be married with Kojou with a daughter named Reina. Yukina had been referred a couple of times as Kojou's princess
Avrora was the Previous 4th Progenitor, and passed down the powers to her servant, Kojou.
Although Kojou cannot recall any of his memories from the time he was with Avrora, he occasionally remember segments from his past.
Avrora Florestina now resides inside Kojous sister, Nagisa, and occasionally shows up throughout the anime, a notable event is when the prisoners of the Rank A prison escape due to the Witch of the Void ''dying'', where she appears in a neko (cat) uniform with the 4th Progenitors 7th Familiar. Nagisa does not remember anything from when she was taken over by Avrora.
He is Kojou's friend and classmate whom he met during middle-school alongside Asagi. He's 16 years old. Motoki is also a member of the Lion King organization and the real observer of Kojou, the fourth primogenitor. He is also Asagi's childhood friend and therefore often encourages her pursuit of Kojou. He's currently going after Koyomi Shizuka and has told Kojou and Asagi that she's his girlfriend, despite having not held her hand according to Natsuki. He is an Esper who have the ability to manipulate special sound waves, but needs certain drugs or enhancers to strengthen his ability. Kojou does not know that Motoki is his real observer.
Yuuma is Kojou and Nagisa's childhood friend. They met when little Nagisa dropped her hat and it ended up hanging on a tree branch that grew on a cliff. Yuuma helped Kojou to retrieve it back.
She is Kojou and Nagisa's mother. Presumed to be divorced to their father and gave Nagisa the race to be a hybrid.
She is a friend of Yukina Himeragi. Sayaka was ordered by the Lion King Organisation to take charge of multinational magical crimes in the Foreign Affairs Division. She was briefly the observer of Dimitrie Valter same just as Yukina was observing Kojou Akatsuki. Sayaka is very protective of Yukina, maybe because Yukina is the only friend of her, as she said in episode 5 or 6. Yukina and Sayaka are both adopted and were roommates. They know each other from their childhood, because Sayaka had a picture of Yukina and hers on her cellphone. She got really mad when she heard that Kojou made Yukina his blood partner. She blamed Kojou for putting Himeragi in danger. She has androphobia, due to which she hates men. But she didn't had any problem with Valter and Kojou (Not that much), so this proves that her fear remains only for normal men ( so much that she even rejected calls from her superiors). Sayaka later starts to like Kojou and discovers that he is the only man she is comfortable with. Also, she got pissed off when she saw La Folia kissing Kojou. But she is always shy around Kojou. In anime, almost every time she communicated with Kojou she always blushed. Later, Kojou drinks Sayaka's blood, she starts to like him more. She even added Kojou's phone number in the favorite in her cell phone. However it is not revealed if Kojou has feelings for her.
She made her debut in Episode 11 of the anime and is the eldest Princess of the Aldegyr Kingdom. Sayaka briefly mentioned about her in Episode 9 when she told Kojou that she was sent as an observer of La Folia. La Folia was seen by Kojou and he first thought her to be Kanase. Later she saves Kojou and Yukina from Automatons of her as well as their pursuers. She was trapped on the same island as Kojou and Yukina and later helps them save Kanase who she reveals is her aunt. When Kojou was in coma, she was willing to let Kojou suck her her blood and was going to take off her clothes for him but Himeragi stopped her from doing so. But when Kojou comes back, she solicits Kojou to drink her blood because it was the only way to summon the Third Familiar. She was attracted to Kojou as is evident by the fact that she was going to take her clothes off to allow him to regain consciousness. Before going back to Aldegyr, she bids goodbye to Yukina and Kojou and kisses Kojou much to the dismay and anger of Sayaka, Yukina and Asagi who were present there.
She was a girl who was experimented upon by her adoptive father who had almost turned her into an artificial angel. La Folia revealed in Episode 11 that she was her aunt, as she was a child of her grandfather who had an affair with a Japanese woman, that at the later part of Amphisbaena arc was identified as Kanase Kensei's sister. She calls Kojou "Brother."
She is a teacher at Kojou's school and also the Witch of the Void. Koujou always calls her "Natsuki-chan," probably due to Natsuki's body size and childish face. Although, this somewhat insulting nickname doesn't mean that Kojou respects her less than any older figures in his life, since Natsuki is the only adult that he had explicitly respected, as shown by the OVA.
She was an artificial life form who had a Familiar installed in her. Kojou saves her and she later becomes a maid to Natsuki.
She is the vessel of Lilith, the world's strongest succubus. The bullying Yume had gone through because of her mind-control magic made her think that her existence will always cause problems and harm to others, and she could not kill herself in a simple way because Lilith would simply find another suitable vessel and that person would also have to suffer. So she decided to assist Taishikyoku in their plan to control Leviathan, the strongest living weapon from the ages of the gods as she is the being with the strongest mind-control magic, only to betray them and reveal her true goal which was to die inside Leviathan, which has the ability to dissipate any entity into nothing and which will kill her along with Lilith. But Kojou saves her and gives her a meaning to her life. She took a liking to him and now dreams of marrying him.
In their brief interaction in Alchemist arc, both of them can interact with each other, with Kojou taking the tsukkomi role to Nina's mischievous personality.
It is assumed that they have a loving father-daughter relationship. Daughter of Kojou Akatsuki and Yukina Himeragi. She is similar to Himeragi but has big breasts. She has blue eyes like Kojou and is also a vampire.
Kojou is wary of him, as Kojou sees him as someone that's ruthless and doesn't care about the well being of Itogami Island at all, doing thing merely in the pursuit of pleasure with questionable methods. He can't understand Vatler's way of reasoning, just like the majority of the cast. However, Vatler's over-friendly gestures towards him sometimes make people misunderstand their true state of relationship, thinking that he's close to the older vampire. However, in volume 15, Kojou admitted to Vatler that despite the latter's tendencies to create problems in Kojou's life, he can't really "hate him to the bone."
Kojou seems quite relaxed with Kira, as the latter doesn't intimidate him and always treats Kojou and his friends nicely. His relaxed demeanor around him even made Asagi thought that Kojou is into men.
Kojou doesn't have a civil relationship with Tobias the way he has with Kira. This could be attributed to Tobias' belligerent attitude towards him, that he showed freely by throwing insults to Kojou here and there when Kojou experienced some difficulties, that in Tobias opinion translated as "incompetence."