In Lyddie, who was Lyddie's antagonist?

The antagonist is Mr. Marsden because he leered at Lyddie, forced himself on her, and got her fired.

An antagonist is an enemy, or the opposing force a character faces.  Lyddie’s first antagonist is probably Mrs. Cutler, because she is a horrible boss.  However, she is not at Cutler’s Tavern very long.  Although Mrs. Cutler makes life miserable for Lyddie and then fires her for leaving even though she had permission from the cook, she moved on from that experience.  Unfortunately, she found herself facing an even worse boss at the factory.

The factory system was very oppressive to begin with.  The girls had to live in the boarding house and do what the company told them, including dress a certain way and attend church.  Lyddie’s boss at the factory was a leering, completely inappropriate man.  When she was sick he forced himself on her when she couldn’t defend herself, trying to kiss her almost before she realized what was happening.

She tried to stare him down, but her eyes were burning in their sockets. Let me go! She wanted to cry. She tried to pull back from him, but he clutched tighter. He was bringing his strange little mouth closer and closer to her fiery face. (Ch. 16) 

Lyddie stomps him and stumbles away.  After this she is in a fever for several days, and nothing comes of the incident.  By the time she returns to work it is forgotten.  However, she still feels uncomfortable around him because of the way he stares at her. 

One day she sees Mr. Marsden and her friend Brigid.  He is trying to do the same thing to her.  Lyddie does not think.  She just hits him with a bucket. 

At the sound of her hoarse cry, the overseer whirled about.  She crammed the fire bucket down over his shiny pate, his bulging eyes, his rosebud mouth fixed in a perfect little O. The stagnant water sloshed over his shoulders and ran down his trousers. (Ch. 20) 

This time Lyddie is fired, and she accepts this.  He accuses her of being immoral, which is ironic since he is definitely the immoral once.  However, she does write a letter to Mr. Marsden’s wife exposing him and gives it to Brigid.  She tells her she must promise to give it to his wife if he targets her again.  It is the best she can do to protect her and the other girls from this predator.