Same word as vanquisher:

advocate, ally, backer, challenger, champ, conqueror, defender, endorser, exponent, expounder, guardian, hero, heroine, medalist, nonpareil, number one, numero uno, paladin, partisan, patron, proponent, protector, supporter, sympathizer, the greatest, titleholder, top dog, upholder, vanquisher, victor, vindicator, warrior, winner