(n.) One who, or that which, impels.

Maecenas accessory accomplice accomplice in crime actuator admirer advocate aficionado angel animator apologist backer buff cajoler champion coax coaxer coconspirator cohort confederate conspirator defender dependence encourager endorser energizer exponent fan favorer fellow conspirator firer friend at court gadfly galvanizer impeller inducer inspirer lover mainstay maintainer mover moving spirit paranymph partisan patron persuader pleader prime mover promoter prompter protagonist reliance second seconder sectary sider socius criminis spark spark plug sparker sponsor stalwart standby stimulator support supporter sustainer sympathizer tempter upholder votary well-wisher wheedler