Prosopagnosia, also known as face blindness, is a mental condition which prevents the afflicted from recognizing faces, including their own. It plays a major role in Nine Hours, Nine Persons, Nine Doors, as Gentarou Hongou suffers from this condition and partially uses it as motivation to create the First Nonary Game. It is unclear which type of prosopagnosia Hongou has.
Prosopagnosia can be categorized into two types: Development and acquired.
Development prosopagnosia occurs when the brain is unable to distinguish between people's faces, and their features. This leads to the person with the disorder having trouble remembering a person's face. This even occurs with members of their family.
Acquired prosopagnosia is different from developmental prosopagnosia in that the person with the acquired prosopagnosia is able to identify the features of the face but cannot determine who it is.
Nine Hours, Nine Persons, Nine Doors
While in the Laboratory, Junpei and Lotus talk about the morphogenetic field as Lotus hacks a computer. Lotus mentions that it may be caused by diseases such as amnesia, aphasia, blindsight, and prosopagnosia. Junpei asks what prosopagnosia is and she describes it as being a condition which makes one be unable to recognize human faces.
Later in the Cargo room, Ace struggles with a simple puzzle which involves matching people's faces to a box with their number. Ace claims he is tired and cannot recognize the faces. Remembering what Lotus said, Junpei realizes Ace has prosopagnosia. However, he feels bad for Ace and decides to say nothing of it.
Later, after Clover and Snake are killed, Junpei realizes who their killer is. He tells Ace that he is Santa, but Ace calls him out on his lie. He asks why Ace believes he is Junpei and Ace says that it's because Junpei's bracelet is 5. Junpei finds this answer odd, as most people would simply say that they didn't have the same face. Junpei then publicly concludes that Ace has prosopagnosia. Ace reluctantly admits it.
Junpei then goes on to explain that he believes the killer has prosopagnosia, as Snake is not actually dead. This corpse is someone unknown. Junpei calls this man "Guy X". He further explains that the face blind killer was tricked into believing Guy X was Snake because Guy X was wearing Snake's clothes. Backed into a corner, Ace confesses to murdering them.
Prosopagnosia is only mentioned in the Incinerator by Junpei after Ace talks about his condition. Junpei remembers that Ace has prosopagnosia from the Safe Ending because young Akane Kurashiki gives him that information. Ace also mentions his prosopagnosia during the First Nonary Game, by telling young Akane, "You don't know what it's like to spend every day surrounded by monkeys."