Thomas, the main protagonist in An Enemy of the People, has recently returned to his hometown to work as the chief medical officer for the medicinal Baths. He was the first to propose the Baths as a source of income for the town, having thought of the concept while working in the north. The play does not discuss why he left for the north in the first place, but his former status as a low-earning rural doctor sets him apart from his brother, who has stayed in their village and risen to the rank of mayor.

Thomas is a principled man who, throughout the play, stands up for what he knows to be the truth. An intelligent scientist, he realized that the Baths were potentially dangerous long before he had direct scientific proof. He is also an idealist, and at first he trusts all other characters to do the right thing; as soon as he knows for sure that the Baths are dangerous, he assumes they will be fixed no matter the cost. This idealism causes him to be somewhat naïve to the motivations of other characters. He takes their support at face value and does not understand when others appear to care more about money or public opinion than about poisoning the Baths’ visitors.