Kazuha and Heiji Hattori have been close friends since childhood. Their fathers work together at the Osaka Police station, where Kazuha's father, Ginshiro Toyama is a close friend and partner of Heiji's father, Police Chief Heizo Hattori. Her mother is mentioned in episode 573, where she wanted Heiji to bring Kazuha her drinkbottle.
At one time in their youth, Heiji had had the unfortunate idea of locking themselves together with a set of old handcuffs which had no key and could not be unlocked; as a result they had to involuntarily spend time together even on some rather intimate occasions (i.e., bathroom visits) before the cuffs could be removed. As a reminder of this 'closeness' and for good luck, Kazuha fashioned two identical lucky charms, each incorporating a chain link from the cuffs, which she and Heiji have been wearing ever since. At some later point, Kazuha added a picture of Heiji into her own charm pouch.
After Heiji met Conan and discovered his true identity, he talked about him so frequently and enthusiastically to Kazuha that she became convinced that Heiji was in love with a Tokyo girl named 'Kudo'. When Heiji invited Conan, Ran and Kogoro to Osaka, Kazuha spied on them, believing Ran to be 'Kudo.' At an okonomiyaki restaurant, when Heiji got up for a moment, Kazuha sat down in his seat and warned Ran to back off him, at which point Heiji returned, the misunderstanding was cleared up, and Heiji laughed at Kazuha's behavior. In their next meeting, Kazuha continued to treat Ran coldly, pointing out that Ran and Heiji's clothes had coincidentally matched the last few times they had been together. In response, Ran immediately changed clothes, after which point the girls became much friendlier.As time passes, Kazuha and Ran grows into close friends, and Kazuha always sleeps in Ran's room whenever she stays in Tokyo.