Which part of the personality might be described as "the great compromiser"?

The answer is "The ID".

Freud saw the personality into three sections, the id, ego and superego, all creating at various stages in our lives.

The id is the piece of our mind which reacts specifically and instantly to the impulses and instincts. The identity of the infant kid is all id and just later does it build up an ego and super-ego.