Kinji is Kinichi's sibling who he loves very much. He cares for him even though he left Kinji with a lot of trouble after his sudden leave of absence from his life. To Kinji, Kinichi was like a god who he could always trust and was his only family that raised him after their parents passed away before Kinzou and Kaname were known to exist. However, after the events of Volume 4, they have been on bad terms. Even so after all that has happened and even having to fight sometimes, they still love each other as siblings and will try to help Kinji (even if it is indirectly). Also its been said many times to Kinji's disbelief that Kinji has a power that one day will surpass Kinichi's own.
During Kinichi tenure while being a double agent in IU he would meet Patra and would begin to fall in love with her. After, Kinichi is shot in the heart twice by Sherlock Holmes, Patra is able to heal him where they hugged. Sometime after the Far Eastern Warefare (FEW) came to an end, they would get married and conceive a child together.
Kinzou Tohyama (GIII) is Kinichi younger genetic half-brother. In Volume 11, it is revealed they have some kind of understanding with each other.
Kaname is Kinichi younger genetic half-sister, who he helped heal after Kinzou (GIII) attacked her and left her in a near death state.
Magane is Kinichi grandfather.
Setsu is Kinichi grandmother.
Meiya Romano was Kinichi's amica while he was studying at Rome Butei High for two years and taught him recitations from the Bible, which he would memorize through his HSS abilities. During the two years, the two would get along very well to where they were on the same wavelength and would hunt down criminals together. However, Kinichi never told Meiya his real name and sex making her believe his name was Kana and he was a woman.