OCD

Fans and scholars have retrospectively diagnosed the character as being on the spectrum, and you can easily find essays and discussions as to whether Poirot’s fastidiousness at least borders on OCD.

Nicholas has autism, anxiety, OCD, and says he has always experienced depression.

He opened up about living with anxiety in his twenties, saying: "I didn't know who to turn to," adding: "I'm running to raise money for people who are affected with loneliness, anxiety, depression, OCD, insomnia, whatever it may be."

He described his “pure O” in a February interview on Australia’s version of “60 Minutes,” explaining that he focuses almost completely on the thoughts that are typically associated with OCD, rather than exhibiting any visible compulsive behaviors such as turning lights on and off, checking and rechecking locks or hand-washing excessively.

According to Healthline, pure obsessional disorder is not a separate diagnosis from OCD, but the specific description does apply to many people.