We'll put the specifics after the break in the interest of protecting your delicate eyes from (unconfirmed) spoilers, but we're pretty sure this will be a central (read: widely advertised) aspect of this still-unannounced game anyway.
Siliconera reports that Isaac is accompanied on-screen by a character called "Shadow Isaac" -- a second personality who appears on-screen as a distinct character. This Shadow Isaac entity frequently taunts "real" Isaac, mocking him for his mistakes. The two even carry on conversations as the missions progress.
It's like Fight Club in space.