co-signer
US /ˈkoʊˌsaɪ.nɚ/
UK /ˈkoʊˌsaɪ.nɚ/

1.
codeudor, avalista
a person who signs a document, especially a loan agreement, jointly with another person or persons, and thereby guarantees payment or performance
:
•
My parents had to be a co-signer for my first apartment lease.
Mis padres tuvieron que ser codeudores de mi primer contrato de arrendamiento de apartamento.
•
Without a co-signer, I wouldn't have been approved for the car loan.
Sin un codeudor, no me habrían aprobado el préstamo del coche.