Tooyama Kazuha is the childhood friend and romantic interest of Hattori Heiji. Their fathers work together at the Osaka Police station, where Kazuha's father, Tooyama Ginshirou is a subordinate and close friend of Heiji's father, Police Chief Hattori Heizou.

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 to unlock it; as a result they involuntarily had to 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.

Kazuha has a hot temper and can be highly critical of people around her. She has been hinted to have a high level of intelligence, as she was able to deduce a dying message in an episode even more quickly than either Ran or Kogoro could (albeit with slight help from Heiji). Moreover, she is highly confrontational and unafraid to let others know of her dislike, even showed no hesitation or concern confronting Heiji in men's restroom. This trait leads to frequent shouting matches with Heiji, but it also allowed her and Ran to forge a strong friendship.

She also has a marked belief in spirits, good-luck charms, and other superstitions. Right from her first experience, she displayed her belief in the importance of hers and Heiji's charm, which Kazuha made to protect her and Heiji from harm. And indeed, during the same case, Heiji lent Conan his charm, and the metal inside it stopped a knife blade that would have killed Conan. On subsequent cases, Kazuha is quick to attribute mysterious shadows and strange events to ghosts and other malignant spirits, which usually sends Ran into a panic as well, though she herself does not possess a morbid fear of ghosts as much as Ran does.