Kazuha Tooyama

Heiji Hattori and Kazuha Toyama grew up together and are easily each other's best friend and mutual love interest. They have gone at the same school since they were in pre-school. Neither of them have been shown to have other close friendships outside of their relationships with Shinichi and Ran, though Heiji does have a rivalry with fellow kendo practitioner Okita. They often spend time together, and Kazuha typically comes along when Heiji visits the Mouri's or vice versa, and is usually getting caught up with Heiji in criminal investigations. Kazuha and Heiji both wear twin good-luck charms that Kazuha made with pieces of the handcuff chain that stuck them together temporarily as children. Kazuha and Heiji have both shown jealousy when the other appears interested in someone else, even though they're not officially in a romantic relationship. Although, all their Jr. High School and High School classmates have commented that they're a couple. Their parents have known each other before they were even born. Making their families close.

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.