(n.) One who seconds or supports what another attempts, affirms, moves, or proposes; as, the seconder of an enterprise or of a motion.

Maecenas abettor admirer advocate aficionado angel apologist backer buff champion defender dependence encourager endorser exponent fan favorer friend at court lover mainstay maintainer paranymph partisan patron promoter protagonist reliance second sectary sider sponsor stalwart standby support supporter sustainer sympathizer upholder votary well-wisher