Aunt Lydia is a major antagonist in Margaret Atwood's novel, The Handmaid's Tale.
Aunt Lydia seems to be a high ranking aunt in the republic of Gilead. It seems that earlier in the novel, she has been one of the aunts in charge of Offred's facility, along with Aunt Elizabeth. She seems to be one of the most instructive of the aunts. Throughout the novel, Offred is constantly reminded of her teachings and advice.
At one point in the novel, she reveals to Janine how Moria has escaped from the facility. She seems to expect Janine to tell the other handmaids, and they in turn, would no better than to make the same mistake and try to escape.
She eventually seems to leave the facility, and Offred does not see her again for a few years it seems. The next time Offred sees her, it is at a Salvaging execution event, in which Aunt Lydia overseers the execution, and displays the convicted. This is the last time in the novel that she is seen.
Through these actions, it can be assumed that Aunt Lydia intends to use fear to educate and control the handmaids.
The aunts are the women who are put in charge of teaching the handmaids about Gilead, and making them learn proper customs and rules. They are equipped with cattle prods to strike the handmaids with if things should ever get out of hand. They try to get handmaids to be grateful and honored for their role in Gilead, and make them see the downplay in acts against the republic, such as gender crime. Aunts have also been known to make the handmaids watch pornographic films to show them what the world was like before the Republic of Gilead, and make them think that things are better now.
It is likely that many aunts were once handmaids themselves, and were promoted to aunts when they became infertile. After becoming infertile, the handmaids could either become marthas, wives, or aunts. Those who chose aunts became responsible for training other handmaids.
Symbolically, the aunts could represent those who have completely and easily submitted to a totalitarian state.