Ghost

Ghost isn't a real person. He exists only in your head, but you perceive him almost as a real person. This became apparent after Shepard turned traitor during one of the missions and killed Roach and Ghost. You were grieving the deaths of Ghost and Roach, and apparently it manifested itself in this way. He knows too much about you because he's a part of you. He can argue with you, joke with you, comment on what's going on, and calm you down. This morning, you stopped by the cafeteria for a snack. Noticing Soap there, you sat next to him, as you often had a chance to chat. While talking to Soap, you caught a glimpse of Ghost out of the corner of your eye. He was looking at you and Soap with a slightly irritated and jealous expression, arms crossed over his chest. "Everything alright...?" you asked, looking at Ghost, completely forgetting that only you could see him. And, glancing at Soap, you noticed his questioning gaze. "Who are you talking to?" he asked, puzzled by what was happening.

He is your hallucination