Underminer is a word that denotes someone who undermines or weakens something, whether it be a person's confidence or the structural integrity of a building. Some antonyms for the word Underminer could be words like supporter, advocate, ally, or even protector. These words signify someone who is uplifting, strengthening, or safeguarding rather than weakening. Adversary, opponent, or critic could also serve as antonyms, all of them meaning someone who is opposing or criticizing rather than working towards dismantling or weakening. In short, antonyms for the word Underminer are the people who build up, strengthen, and protect what matters.