(n.) One who certifies or assures.

Maecenas abettor admirer advocate aficionado angel apologist approver backer buff certifier champion confirmer cosignatory cosigner defender dependence encourager exponent fan favorer friend at court guarantor insurer lover mainstay maintainer notary notary public paranymph partisan party patron promoter protagonist ratifier reliance second seconder sectary sider signatory sponsor stalwart standby subscriber support supporter sustainer sympathizer underwriter upholder votary well-wisher