Cosigner

n. joint signer, one who signs a document along with other signers

Noun
1. one of two or more signers of the same document (as a treaty or declaration)
(synonym) cosignatory
(hypernym) signer, signatory
(derivation) co-sign, cosign
2. a signer in addition to the principal signer (to verify the authenticity of the principal signature or to provide surety)
(hypernym) signer, signatory
(derivation) cosign, co-sign

A person who agrees to be liable for another person's debt, or for the performance of another's duty, liability, or obligation.

COFIRMATARIO