Antagonist
Character Role Analysis
Society
There isn't really a villain in this book at all. Yes, we're rooting against Mrs. Herriton, but that doesn't necessarily make her the antagonist. Sure, she's controlling and at times manipulative, but that's partly due to the way she's been raised within confines of Edwardian society. The closest thing we get in terms of opposing forces is Society, which fights against forces like love and freedom.
Lilia's attempt to go against society leads her to temporary freedom and happiness with Gino, but eventually, she is forced to realize that going against societal expectations has its consequences. Along the same lines, society pressures Mrs. Herriton to help Lilia's baby (even though she has no desire to), and this leads to tragedy and death.