Dr. Thomas Stockmann
Dr. Stockmann is gregarious, impulsive, and careless of appearances. He easily embraces new ideas and has a hard time understanding why other people don't see things the same way. He is convinced everyone will appreciate his discovery about the Baths and he is shocked and disillusioned when they do not.
The mayor is careful and inclined to save money whenever possible. He thinks his brother is too careless and inclined to behave in a rash manner, and they do not have a good relationship with each other. The mayor also acts in his own self-interest at all times, and he can't understand why Dr. Stockmann would do otherwise.
Hovstad expresses radical ideology, but he doesn't live it. He prints "safe" and predictable stories in his newspaper and tries to please the mayor and his readers whenever possible. He may only have supported Dr. Stockmann because he wanted to curry favor with the doctor's daughter, Petra, who rejects him.
Petra adores her father and will do anything to support him. She truly believes in the radical principles to which people like Hovstad only give lip service. She seems oblivious to the interest of the men like Hovstad and Billing. She rejects Hovstad's advances and is shocked he would offer to help her father only as a way to woo her.
Aslaksen is proud of himself for his standing in the town, which he believes is a result of his moderation in all things. Although he expresses a belief in more revolutionary principles, Aslaksen claims a person who owns property can't afford to risk too much—an idea Dr. Stockmann totally rejects.
Katherine Stockmann is a loyal wife and a caring mother. She tries to support her husband, but she worries the family will suffer because her husband is so impractical. When it is evident her husband's research is going to get him fired, she attempts to stop him out of fear for how the family will survive. When she sees how everyone in town turns against him, though, she stands up for him.