Absolutely every boss in Metal Gear Solid is very well-developed, with their own story and unique abilities. Psycho Mantis is no exception. He uses music to control the mind and can completely manipulate a person. When players first encounter Psycho Mantis, he takes control of Meryl. Snake doesn't understand what's going on and how to deal with him, especially considering that Psycho Mantis reads his mind and knows his every move.