Officer Sugimoto

Sugimoto  (杉本 巡査, Sugimoto Junsa, literally "Policeman Sugimoto") is a rookie police officer who served under Yamagiwa and Minoura, also being the former's killer.

Appearance
Sugimoto is a young man with short, choppy brown hair and brown eyes. He wears a standard police uniform, and is considerably shorter than Minoura.

Personality
Sugimoto is largely skittish and nervous as a rookie police officer, however depicts and earnest and eager spirit in improving his skills, and greatly respects rules and protocol.

However, in the past, having failed entrance exams three times, he became desperate and involved himself with a shady politician in order to enter the police force. When forced to kill his lover and coworker, Yamagiwa, he initially begs her to stop her investigation, and goes to the drastic measure of threatening suicide should she not comply. Sugimoto deals with heavy feelings of guilt and paranoia after killing her when she tries to stop him. His love for her was genuine enough that he could not go through with making her death resemble the Port Mafia's modus operandi.

Sugimoto is quick to act rashly, but gives up just as easily.

Background
Sugimoto had failed the police entrance exams three times. At this point he was already on the brink of giving up, but then a certain politician had approached him and aided him in getting into the police force. In exchange, Sugimoto would have to follow his orders.

Sugimoto had been in a relationship with Yamagiwa.

Plot
Sugimoto had been contacted by the politician which he had been working for to get rid of the evidence Yamagiwa had against him. Sugimoto had called Yamagiwa out early in the morning to talk about it.

Yamagiwa had refused to hand over the evidence and so Sugimoto had threatened her with a gun. When Yamagiwa had known Sugimoto could not shoot her, he changed his threat to shooting himself in the head.

Yamagiwa had panicked at this and tried to stop Sugimoto. In the process, he had accidentally killed Yamagiwa by shooting her in the chest.

The diet member had then told Sugimoto how to cover up his crime.

He appears on scene where Yamagiwa's body is recovered from the river, alongside his boss, Ranpo Edogawa, Osamu Dazai, and Atsushi Nakajima. Although appearing skittish, he otherwise shows no signs of being the prime suspect.

However, with his deductive reasoning, Ranpo determines Sugimoto is in fact Yamagiwa's murderer, a claim that greatly upsets both police officers. When it's apparent Ranpo's deduction cannot be countered, Sugimoto attempts to shoot, but is pinned down by Atsushi.

Sugimoto is later brought to an interrogation room, likely arrested for his crimes.

Trivia

 * Sugimoto is a police detective in Ranpo Edogawa's work Monster Trick.