Yamagiwa was an officer who was murdered, whose body was found floating in the river. The case of her murder was investigated by Ranpo Edogawa of the Armed Detective Company, along with Atsushi and Dazai.
Yamagiwa was a beautiful woman appearing to be in her early thirties. She had a set of wide hazel eyes which match her long brown hair.
Yamagiwa was a brave and honest officer, willing to go into great lengths to investigate a powerful corrupt politician. She never backed down from her investigation despite knowing the dangers of her pursuit.
She also loved her lover - Junsa Sugimoto - greatly, even going as far as sacrificing her own life in order to save his.
Yamagiwa was a police officer that had been investigating a certain politician due to corruption. During her investigation, she had come across information regarding a diet member.
It is later discovered that she was involved in a romantic relationship with Junsa Sugimoto, a sergeant in the police force as well as her subordinate. However, the nature of their relationship had been kept a secret at work.
Yamagiwa's body was found in the river after she had been shot thrice in the chest by an unknown assailant. The case of her murder was handed to Minoura, her superior in the police force.
The police found difficulty in continuing with her case, as they could find no evidence related to the homicide nor do they have any suspects, leaving them with no leads to pursue.
The difficult nature of the case prompted the police to call for assistance from the Armed Detective Company, who sent Ranpo Edogawa to investigate the case.
He activated his Super Deduction ability, and with it deduced within mere seconds that the murderer of Yamagiwa was none other than Sugimoto, her subordinate as well as an officer working on the case of her murder.
When brought to the police station for questioning, Sugimoto revealed that he was a mole placed in the force by the corrupt politician whom Yamagiwa was investigating. He was tasked with getting rid of the evidence that she had obtained, and would be killed by a hit man should he fail the task. In order to force her to hand over the evidence, Sugimoto threatened to kill himself in front of her. Yamagiwa tried to prevent this, which caused her to be shot by accident instead.
In panic, Sugimoto contacted the politician for help in getting rid of the evidence. He was told to shoot Yamagiwa thrice in the chest, as well as kick her head into the pavement to make it look like the Port Mafia had murdered her. However, due to their former relationship, he only ended up shooting two extra shots into her chest before dumping her body in the river.