Companions in Mass Effect are special NPCs that can be recruited to join the player's party. Companions fight alongside Commander Shepard, and have their unique set of stats, abilities, equipment lore and even their own personal Assignments. Each Companion have their own agenda and personality, this allows them to agree or disagree with you depending on the choices you decide to take throughout the course of the game. Some Companions can be recruited if certain conditions are met.
Mass Effect Companions Guide