The Protagonist is the main character of Lost In Vivo.
The protagonist's gender and appearance is left ambiguous.
The protagonist is told to undergo "In Vivo exposure therapy" by their therapist/psychiatrist during the dialogue 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.
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.