Kazuki's young brother. Kazuki couldn't help with how Yuuji was treated despite Kazuki is a superior among the others and took care of him better among the rests prior to his childhood. Her love towards Yuuji grow stronger even reaching to how a lover would be, even shown to be jealous when she learned that Yuuji befriended with a girl and the latter had rivalry with Mihama girls for taking a photo together. Her love towards Yuuji is not changed even after she finally meet Yuuji again after her absence due being taken to become a Thanatos' vessel. In the visual novel, it was revealed that Kazuki even had sex with him, possibly the first one who taught him about it.
Kazuki initially felt nothing towards Amane during her life before the accident happen. Amane's helpless towards the situation and always depending on Kazuki grew a great friendship between them, especially since Kazuki viewed Amane just like Yuuji. After the reunion to save Yuuji, Kazuki glad to meet her well directly and ended up having a love rivalry to her since she noticed that Amane has feelings towards Yuuji. Amane is the only girl of Mihama Academy to call Kazuki by her name after learning her identity as Thanatos.
Kazuki grew close with Yumiko as she is the closest one to be in contact during the operation to rescue Yuuji. Kazuki even once teased her for having her to confess that she is in love with Yuuji, though Kazuki was surprised that Yumiko able to do so without feeling ashamed until Yumiko is when the girls were smiling at her.
Kazuki viewed Michiru as an idiot beyond help and during the plan to get money income, she only told Michiru to do as she said. After the incident with Heath was over, Kazuki helped Michiru with her study and sighed over for hearing Michiru misspelled her Thanatos name.
Kazuki had Sachi to collect items to make smoke grenades and accompanied Makina to get her rifle. It is unknown if Kazuki noticed that Sachi is the Sachi who Yuuji had told during their childhood as they never met.
Kazuki bore a great hatred towards her father and educated Yuuji to never become like him. Ryouji himself actually dislike Kazuki and only depended on her intelligence. Unfortunately for Kazuki, the more she scolded Ryouji for what he had done to Yuuji, the more Ryouji hate Yuuji. It was of his violence that made Kazuki decided to be inferior by having a normal school life.
Kazuki disliked her mother, but not hating her as she knew that Satoko was only a helpless woman. Unfortunately, Kazuki never know how Satoko able to care Yuuji so much after Kazuki's disappearance.
Kazuki appreciated Chizuru's responsibility on taking care of Mihama girls and Yuuji to have him feel an ordinary school life. Because of her achievement, Kazuki decided to have Chizuru to be a principle again in the new Mihama Academy she built. Just like Yuuji, Kazuki seemed to like on teasing Chizuru.
Kazuki knew her during her time as Thanatos even on how close she is towards Yuuji, the reason of Kazuki's trust to have Yuria to help her on saving Yuuji and help on building the new Mihama Academy.
CIRS:[]
CIRS took care of Kazuki as Thanatos. Kazuki was a secret commander after becoming Thanatos, only giving command when in need. Yuria labelled her as an "Underground Professor" to anyone who has no official stand to face with her.
After learning that Heath was trying to take over Thanatos, Heath become Kazuki's enemy. Heath also noticed about this and thru unknown method, he took a bit of her cell to be used on cloning the 'other' Yuuji to have her intelligence.
Robert was one of the researcher who looked up on Kazuki during her awakening as Thanatos. Kazuki manipulated Robert so she gained her memory slowly during her amnesia.
