(See
Metal Gear Solid)
Former KGB psychic with powerful telekinetic and telepathic abilities. He was once asked to read a serial killer's mind, but he went a little bit too far into that mind...
He has a black suit, and wears a gas mask all the time - probably better so, because his face isn't that pretty.
He's now a member the Fox-Hound, but he has goals that are quite different from Liquid Snake's plans.
NOTE: SPOILERS AHEAD
His powers are actually so fearsome that they even go over the boundary between the game and reality. In Playstation version of Metal Gear Solid, and in GameCube Metal Gear Solid: The Twin Snakes, he's able to see what you have saved to memory card. He also makes the rumble feature on the controller vibrate. (See Coolest fourth wall breaches in computer and video games for a more detailed discussion of this insanity!)
Regrettably, the only joke Konami has left to the PC version was related on how you can beat Psycho Mantis. Mantis can read your controller, of course, as said above - but he can only read the first controller port! So, to prevent him from reading your mind, you need to plug your controller pad to second port (or, in PC version, you must play with keyboard instead of using a pad). In GameCube, switching to any other port will work.