William Petenshy

William Petenshy was a fancily dressed man who was seen having an argument with Decago Mieterman outside of Sōseki Natsume's residence during Ryūnosuke Naruhodō's investigation into Viridian Green's attempted murder.

An escaped criminal
Petenshy was a prisoner who looked after one of his fellow inmates named Seldan, who was to be executed. Wanting to reward his loyalty, Seldan told Petenshy that he had hidden a treasure in his old flat at John Garrideb's complex and gave him the key with which to open the treasure. Petenshy later somehow escaped and went to the complex, but the flat on the second floor was already occupied by Duncan Ross. Thus, Petenshy, desperate to obtain the treasure, settled in the flat on the first floor.

Petenshy plotted to force Ross to leave his flat by putting gasoline in the lamp line. Unfortunately, Ross died of gasoline-induced asphyxiation instead of fleeing his room, so this led people to believe the flat was haunted and that Ross's death was an accident. Petenshy could not enter the flat, however, because of the investigation of Ross's death, and by the time it was done, Sōseki Natsume had moved into the flat.

A fiancée's revenge

 * Main article: The Adventure of the Clouded Kokoro

Ross's fiancée, Viridian Green, heard about his death and became depressed. Seeking revenge, Green bought strychnine off the black market and sent a letter to Petenshy to meet her at a nearby pub, saying that she knew vital information about Ross's death in order to lure him out of his bedroom. He followed the instructions of the note, which allowed Green to put the strychnine in the same lamp line, but not long after, she was accidentally stabbed by Joan Garrideb. An investigation was thus commenced regarding the stabbing, Natsume being the prime suspect, so Petenshy was barred from entering Natsume's flat and thus had no reason to fiddle with his gas.

During the investigation of said stabbing, Petenshy got into an argument with Decago Mieterman, who was sent by his boss's wife, Mrs. Altamont, regarding Petenshy's gas payments.

The truth of the poisoning

 * Main article: The Memoirs of the Clouded Kokoro

Personality
Petenshy wore flashy pink clothing that looks remarkably out of place when compared to those around him. He seemed to have a very high sense of self and frequently references Shakespeare in his speech, as evinced by his calling others "Horatio" and reciting the "sound and fury" soliloquy after being awakened from his poisoning. He acted very antagonistically toward Mieterman.

Name

 * The names of Petenshy and Decago Mieterman are not given during Dai Gyakuten Saiban: Naruhodō Ryūnosuke no Bōken. However, the title of "Mysterious Man A" is given to Petenshy in the game's artbook, while Mieterman is referred to as "Mysterious Man B".
 * His surname comes from a Japanese word meaning "crook" (ペテン師), which hints at his criminal background.
 * "William" is a reference to "William Shakespeare" who he often quotes.