1) Admirer 2) Anchor 3) Backbone 4) Booster 5) Bread and butter, so to speak 6) Bulwark 7) Champion 8) Chief support 9) Chief supporter 10) Dependable type 11) Friend 12) Linchpin, principle support 13) Pillar 14) Principal support 15) Protagonist 16) Supporter Found on https://www.crosswordclues.com/clue/mainstay
Main'stay` noun 1. (Nautical) The stay extending from the foot of the foremast to the maintop. 2. Main support; principal dependence. « The great mainstay of the Church.» Buckle. Found on http://www.encyclo.co.uk/webster/M/9