The Protagonist is the main protagonist of the game Lost In Vivo.

The protagonist's gender, and appearance is unknown. Given the subject matter of the game, and several nods toward the player character undergoing some kind of therapy, it can be speculated that the protagonist is someone with a military background that has some kind of underlying personality disorder involving self-image. There are many times when the protagonist is critical of themselves; calling themselves disgusting or ugly when examining the pile of acid dissolved meat, the Dark Mirrors showing a grotesque image of the player back at them, constant references to body shape and low self esteem, the bulimia puzzles, comments by the Brain, and the ending comments regarding the experience trying to teach them something.

The protagonist is told to undergo "in Vivo exposure therapy" by their therapist/psychiatrist during the dialog at the start of the game. This is a real type of cognitive behavior therapy that is used to treat a variety of different behavioral conditions such as simple phobias, PTSD, general anxiety, and other related conditions. It is administered by either flooding, or systematic desensitization. Flooding is the act of rapidly exposing the individual to feared situations, whereas systematic desensitization is the gradual exposure to feared situations coupled with relaxation exercises when stress levels become too great. This information could very well be used to theorize more about the situation the protagonist finds themselves in during the game, and what it could possibly represent. It is also revealed that the protagonist is on Fluoxetine, or Prozaic, which is used to treat major depressive disorder, obsessive–compulsive disorder (OCD), bulimia nervosa, panic disorder and premenstrual dysphoric disorder.

While never explicitly stated, it can be reasonably inferred that the protagonist is a mildly disturbed individual who has found themselves trapped in a set of trials aimed at bettering their character and teaching a lesson through a twisted form of in vivo exposure therapy. It is up to the player to decide if the protagonist will pass (or fail) these otherworldly tests of character, by triggering secret endings or making the "other" choice at the end of the game.