1) Loan signer, at times 2) Secondary loan signer 
A comaker is a person who signs a negotiable instrument with another person person and thereby promises to share the liabilty to make payment on the due date.
One of two or more people who sign the same check or promissory note. Each comaker is liable for the entire amount to be paid. (See also: maker)
a person who formally undertakes to discharge the duties of the maker of an instrument, esp. a promissory note, in the event of the maker's default.