"Mushi-kun, I bet you're laughing at me for destroying myself for the sake of mystery. But if that's the case, maybe there's no such thing as unshakable values. Maybe it's up to us to decide what to put value in and what to live for. After all, we have the right to turn our own decisions into our entire world. It is, foolishly enough, the greatest luxury afforded to mankind."
Yokomizo had curly hair that reached his nape with a slicked portion of his bangs covering his forehead. Prior to his death and during Mushitaro's visions of him, Yokomizo often wore an outfit that consisted of a mandarin collar shirt underneath a three-quarter sleeve yukata tucked into a pair of hakama.
Given that he usually appears as an apparition to Mushitaro, Yokomizo's face is shown as a dark silhouette, though he is frequently depicted with a smile during such instances.
Yokomizo possessed a talent in writing mysteries given his vast expertise in the field of mysteries, knowing writing techniques like Knox's Ten Commandments and for locked rooms. As a result, he became quite famous as a mystery writer known by his pen name Kindaichi (金田一, Kindaichi?). He was also quite detail-oriented to a fault, ensuring that he did not repeat his past mystery storylines and murder cases in his other works. Moreover, Mushitaro once mentioned that Yokomizo always avoided the end of an argument.
Despite his prolific achievements, Yokomizo still greatly pursued the "ultimate mystery", a story he believed would transcend reality. In order to achieve this, he usually received suggestions from his friend Mushitaro, though in the end, he decided to willingly offer his life by becoming a murder victim in his own mystery story, a testament of how committed he was to achieve such goal. This was further fueled by his hatred for regret, wanting to create the ultimate mystery before his life's time limit due to his illness was reached.
Ever since his student days, Yokomizo was a close friend of Mushitaro and profusely wrote mysteries, in contrast to Mushitaro who hated such stories. In his adulthood, Yokomizo became a mystery writer and rose to fame for his works. Mushitaro occasionally visited him to pitch some ideas for his ultimate mystery, though Yokomizo merely rejected his ideas no matter how unique Mushitaro thought his ideas were, since Yokomizo knew they already existed in other works.
At some point, Yokomizo acquired gastric cancer and was given a year left to live. During this time, he met with Mushitaro and sought his help in creating and executing the ultimate mystery, which required Yokomizo as the murder victim and a real killer, the role of which he asked Mushitaro to perform. Eventually, while staying at an inn, Yokomizo wrote three chapters of what would be his swan song work and had them published. Later on, after he penned its fourth and final chapter that would narrate the mystery's resolution, Mushitaro strangled him to death, upon which Yokomizo thanked his crying friend for achieving his ultimate mystery.
Upon completing his task, Mushitaro stole the manuscript of the work's final chapter, in which was written how the murder was committed. Later, Yokomizo's body was discovered by an inn hostess, who, prior to seeing his corpse, encountered a tea-serving doll along the hallway holding a pair of severed human ears on its tray.
With the death of a famous writer such as Kindaichi, the business world reread the first three chapters of his final work in hopes of finding how the murder was committed, but to no avail. People who do not usually read mysteries also became obsessed with the bizarre case of the writer dying in the same method as in his own story. In essence, it had become an ultimate mystery that literally transcended reality. With the criminal having stolen the manuscript for the final chapter, it came at a hefty price given the high demand of the world wishing to know the murderer's identity. In the end, it was anonymously sold to Edgar Allan Poe at a price of ¥20 million.
Later, Ranpo Edogawa solved the baffling case and accurately deduced that the author and the killer were accomplices, rendering it impossible to form a deduction from the beginning. Mushitaro also recognized how important it was to keep such collaboration hidden from the public in order to fulfill Yokomizo's last wish for his final work to be acknowledged as an ultimate mystery. Otherwise, the public may degrade his work to be a farce or a meaningless mystery from even the slightest hints of collaboration between the author and the killer.
Mushitaro then yielded to Ranpo's deduction and turned himself to the police for his crimes, including another alleged murder case. From then on, Yokomizo would appear to Mushitaro as a benevolent apparition, sometimes becoming a source of inspiration for Mushitaro to escape baffling scenarios, visiting him in his dreams, or appearing to him whenever he wrote mysteries. Though Mushitaro typically felt disturbed by Yokomizo's vision in some instances, it is nonetheless implied that he yearns for his friend's company, even writing mysteries he so dreaded if he thinks that it is one of the few ways to find Yokomizo.
- (To Mushitaro Oguri) "I hate regret. So I've done whatever I've wanted to do. Up until now, it's been a satisfying life. But now… I've been given a time limit. […] Before then, I have to complete the ultimate mystery."
- Yokomizo was named after Seishi Yokomizo, a Japanese mystery novelist from the Shōwa period. He was well-known for creating his private detective character Kosuke Kindaichi, the namesake for Yokomizo's pen name in the series.