Short answers, pulled from the story.
Psycho Mantis is a fictional character from the Metal Gear Solid video game series who serves as a psychic antagonist. He is also known by the name Tretij Rebenok and was designed to prove his powers by manipulating the hardware the player used to play the game.
Psycho Mantis first appeared in the 1998 game Metal Gear Solid. His debut moment shattered the fourth wall by reading the player's memory card and speaking directly to the person holding the controller.
To defeat Psycho Mantis, the player must shift the controller from the first controller port to the second controller port on the PlayStation console. This action renders Mantis unable to read the player's movements and breaks his psychic hold.
Psycho Mantis was born as Tretij Rebenok and his mother died during childbirth. His father hated him for the death of his wife, and Mantis discovered this hatred when he used his psychic powers to read his father's mind, which drove him to burn his village to the ground.
The Psycho Mantis boss fight is considered iconic because it required the player to interact with the physical hardware of their console. The scene made the player feel exposed and vulnerable by reading save data and causing the console to appear to disconnect.
Psycho Mantis appears in Metal Gear Solid 4: Guns of the Patriots where his ghost controls the body of Screaming Mantis. A younger version of Psycho Mantis referred to as Tretij Rebenok appears in Metal Gear Solid V: The Phantom Pain where he is manipulated by Skull Face.