Short answers, pulled from the story.
Psycho Mantis is a fictional character from Metal Gear Solid who appears inside Shadow Moses Island. He speaks directly to the player holding the game controller and reads memory card data from the PlayStation console.
Psycho Mantis was first introduced in 1998 during the original release of Metal Gear Solid. He later returned as a ghost controlling Screaming Mantis in 2016 and made a cameo appearance in the 2024 game Astro Bot.
Psycho Mantis appears inside Shadow Moses Island where he sits in a dark room to confront the player. His presence spans decades across multiple titles including Super Smash Bros. Melee, The Legend of Zelda: The Wind Waker, and Metal Gear Solid V: The Phantom Pain.
Hideo Kojima designed Psycho Mantis to read memory cards after watching the 1978 film The Fury for inspiration on psychic power. This decision allowed developers to create an immersive experience that broke the fourth wall by accessing save data from games like Castlevania: Symphony of the Night and Suikoden.
Players must move their controller from port one to port two to stop his psychic reading and render him unable to read movements. In PC ports users utilize keyboard usage instead of physical controller manipulation while later PlayStation console ports require switching connections manually through settings.