From Longman Business Dictionary
guarantorguar·an·tor /ˌgærənˈtɔː-ˈtɔːr/ noun [countableC] FINANCELAW a person or organization that promises to repay a loan if the borrower DEFAULTs (=fails to repay it)The federal government is the ultimate guarantor (=the one that will repay if no one else does) of these loans.