Fumi Kindaichi is the younger cousin of Hajime Kindaichi, with whom she shares a somewhat antagonistic relationship, though she conceals this around others. Her Japanese name is written as 金田一二三, and she is nine years old during her initial appearances.

She first appeared in the anime during the Black Butterfly Murder Case and in the manga during the Murder on Christmas Eve story. Her character was introduced at the conclusion of the Murders in the Demon Dog Forest case arc. She once revealed that her father left for Tibet in search of treasure, leading her to live with Hajime and his family.

Her interactions with Hajime frequently involve bickering, such as when she mockingly called him an "old donkey with no kissing experience" after he referred to her as an immature rascal. This dynamic defines much of their relationship, though she masks this antagonism in the presence of others.

Two decades later, as an adult, Fumi becomes a successful mystery novelist, drawing inspiration from the adventures she shared with Hajime and Miyuki Nanase during her childhood. She also takes a position at a detective agency. Writing under the pen name Kaneda Hifumi, she wins a category of the Osokawa Mystery Awards for her novel *The Hinokawa Legend Murder Case*.

As an adult, she involves herself in investigations like The Ayase Serial Murder Case, where she participates actively in the detection process alongside her cousin. Together with her boyfriend Yuuto, an accomplished mystery writer, she investigates a crime scene prior to official police involvement. This case highlights both her enduring link to mystery solving and her growth into an independent and capable investigator who supports Hajime while operating with autonomy.

A numerical theme connects her name to Hajime’s: his name incorporates the character for "one" (一), while hers contains the characters for "two" and "three" (二三).