There are a couple possibilities for this question, all depending on how you want to argue or support your answer. A tragic figure usually is a main character, who usually has at least one major weakness or flaw that brings on much of their suffering, along with the suffering of others. In Ethan Frome, any of the three main characters involved in the story could fit this definition.
Take for example Mattie Silver, the pretty young cousin that comes to help out at the farm. Her tragic flaw is her sweetness and affection for Ethan, which leads to her fatalistic attitude right before the accident. Because she has feelings for him, and the sweetness of spirit to interest him in return, the resulting tragedies occur. She develops a catstrophic attitude--if she can't be with Ethan, and be happy, then she doesn't want to live, and encourages Ethan to take the sled down. Her spontaneous and drastic emotions that are tied up with Ethan lead to much suffering.
Zenobia is also a character that is very flawed, and causes suffering because of it. She is a hypochondriac who craves and seeks attention through her illnesses. This drives Ethan to great dissatisfaction with his marriage, which turns him to Mattie, which leads to the accident, etc. Zenobia's manipulative suffering is a tragic flaw that creates an atmosphere that is perfect for disaster.
Ethan Frome is definitely painted as a tragic figure in his crippled state of misery. His tragic flaw could be that he is too indebted to family, and feels too much obligation to them. This brings him home to care for his parents, and won't allow him to leave his wife to seek happiness. It's a good trait, loyalty, but in this case it keeps him shackled in a miserable situation, that, because he won't resolve it in a different way, leads to his tragic and spontaneous decision on the hill that day.
Ethan Frome overall is a very tragic tale of misery and suffering, and any of the main characters involved can be connected to that suffering. I hope that those thoughts helped; good luck!