Family
Nagisa Akatsuki
As she is his sister, he is very protective of her. He was surprised when he learned that Nagisa is a hybrid and Avrora's soul is inside her, but that does not change anything as he still considers her his sister and takes care and protects her from danger.
He is willing to go through great lengths for her; he once risked his life just to retrieve her straw hat that was at the edge of a cliff when they were young. He also cares for her dearly. This is shown in Return of the Alchemist, wherein he sees the almost dead Asagi and loses control of his powers, but quickly stops when reminded of his sister.
Mimori Akatsuki
She is Kojou and Nagisa's mother. He loves her and cares about her and keeps her away from harm as much possible. Presumed to be divorced to their father, gave Nagisa the race to be a hybrid. He doesn't know that she knows about him being the Fourth Primogenitor.
Gajou Akatsuki
Kojou's father. He brought Kojou's little sister with him out of the island. He thought that Yukina was in love with Kojou and stalking him at first. Kojou refers to his father as old man, but still respects him and doesn't hate him because of that incident which made him a vampire.
Hisano Akatsuki
She is Kojou's grandmother.
Friends
Yukina Himeragi
A 14-year-old 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. She despised him at first, but eventually grew to accept him. She is constantly following him everywhere he goes as his observer and as a result, has become very close to him. She was actually sent for this very purpose with the organization intending for her to be his first blood partner and even his mate.
In Strike the Blood EX, they are implied to be married, and have a daughter named Reina.
Asagi Aiba
Asagi is Kojou's classmate and best friend. She has a crush on Kojou, who is oblivious of this fact. In Strike the Blood EX, Asagi had been referred a couple of times as Consultant Supreme. She is also implied to be married to Kojou, and has a daughter with Kojou named Moegi.
Avrora Florestina
Avrora was the Previous Fourth Primogenitor, who 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 Kojou's sister, Nagisa.
Motoki Yaze
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 has 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 and it is unknown what his reaction would be if he became aware of it.
Yuuma Tokoyogi
Yuuma is Kojou and Nagisa's childhood friend. They met when 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.
Sayaka Kirasaka
She is a friend of Yukina Himeragi. Sayaka was ordered by the Lion King Organization to take charge of multinational magical crimes in the Foreign Affairs Division. She is the observer of Dimitrie Valte part time, just as Yukina was observing Kojou Akatsuki. Sayaka is very protective of Yukina, having known each other since childhood. They initially had a rough relationship; she was upset Kojou made Yukina his blood partner, and blamed Kojou for involving Yukina in dangerous situations. She has androphobia (fear of men), which may have affected her early judgement of Kojou. Sayaka later takes a liking Kojou, and allows him to suck her blood to unlock a new Beast Vassal. Similar to Yukina, she gets irritated when Kojou is seen with other girls.
La Folia Rihavein
She 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.
Kanon Kanase
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”.
Others
Natsuki Minamiya
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.
Kiriha Kisaki
she is the dark sword shaman that appears in the season 2 of the anime.
Astarte
She was an artificial life form who had a Familiar installed in her. Kojou saves her and she later becomes a maid to Natsuki.
Yuiri Haba
Shio Hikawa
Yume Eguchi
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.
Grenda
Nina Adelard
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.
Reina Akatsuki
It is assumed that they have a loving father-daughter relationship. Daughter of Kojou Akatsuki and Yukina Himeragi. She is similar to Yukina but is taller and has bigger breasts. She has blue eyes like Kojou and is also a vampire.
Moegi Akatsuki
It is assumed that they have a loving father-daughter relationship. Daughter of Kojou Akatsuki and Asagi Aiba. She has violet eyes, blue from her father and red from her mother, Moegi is also a vampire.
Dimitrie Vatler
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. And is actually now a close friend "
Kira Lebedev Voltisloa
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.
Tobias Jagan
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." Woah!