What is a Co-signer and when do you need it? A cosigner is somebody who can go on the mortgage application with you to help you qualify for more. They add income to your application to support your qualification, but they do not plan to reside in the property with you. Usually, a co-signer is a close family member, usually a parent or an immediate family member, but a co-signer can also be a friend or a coworker or somebody in your life who cares about you and is willing to help when you need a cosigner.