protestor — an expression or declaration of objection, disapproval, or dissent, often in opposition to something a person is powerless to prevent or avoid: a protest against increased taxation.
loser — a person, team, nation, etc., that loses: The visiting team was the loser in the series.
detractor — The detractors of a person or thing are people who criticize that person or thing.
foe — a person who feels enmity, hatred, or malice toward another; enemy: a bitter foe.
opponent — a person who is on an opposing side in a game, contest, controversy, or the like; adversary.
antagonist — Your antagonist is your opponent or enemy.
assailant — Someone's assailant is a person who has physically attacked them.
enemy — A person who is actively opposed or hostile to someone or something.
opposition — the action of opposing, resisting, or combating.
