Cosigner

Someone who signs a loan alongside the borrower and is fully liable if they do not pay.

A cosigner lends their credit history to an applicant who lacks one, which is common for student loans, first apartments and young borrowers' car finance. The obligation is not a backstop but joint liability: the debt appears on the cosigner's credit file, affects their borrowing capacity, and falls on them entirely at the first missed payment.

Her father acted as cosigner on the lease, which made him fully liable for the rent.