Retsu is Kyouko's grandfather. Although she may have some doubts about his methods, she respects his decisions. She tries to avoid showing a close relationship with the Kudou Family and to him by extension as a member of the Fujibayashi family due to her being part of the 101 Independent Magic-Equipped Battalion.
The two share a friendly professional relationship. Kyouko treats Tatsuya like a younger brother whenever the two meet outside of work. Their relationship became somewhat strained when Tatsuya discovers that the Kudou Family and Kudou Retsu, Kyouko's grandfather, is heavily involved in an experiment to test the feasibility of using Parasitic Dolls as weapons in the Steeplechase event at the Nine Schools Competition. However, their relationship was back to normal after the Parasite-doll experiment, since Tatsuya lacks the emotional ability to dislike someone and as Kyouko was willing to support and cooperate him in his mission of finding Zhou Gongjin.
Kyouko and Mayumi know each other due to both being part of the Ten Master Clans. Mayumi calls her "Kyouko-nee".
Of the Grandchildren of Kudou Retsu, Minoru and Kyouko are said to be particularly close, like siblings. Kyouko cares deeply for her cousin, enough to beg Tatsuya to use his Elemental Sight to discover the reasons behind Minoru's condition.
It has also been revealed that they are not genetically cousins, but half siblings instead (genetically they share the same mother).