Madame Pernelle is Orgon's mother. Like her son, she's enamored with Tartuffe. She eats up every word he utters and spits it back out at Elmire, Cléante, Mariane, and the rest of the family. After Orgon is shown what a fake Tartuffe is, he tries to convince her of the truth. Their exchange captures her nature:
No, no, a man of such surpassing piety
Could not do such a thing. You cannot shake me.
I don't believe it and you shall not make me.
You vex me so that, if you weren't my mother,
I'd say to you…some dreadful thing or other.
Madame Pernelle is also one of the few characters we see voice Tartuffe's opinions. Sure, we hear Elmire and Dorine talk about how Tartuffe is always chastising them, but it's Madame Pernelle that gets down to specifics. In Act 1, Scene 1 she pretty much chews everyone out, telling each one of them individually how they "talk too much" (Dorine), "grow more foolish everyday" (Damis), and are "much too free with money" (Elmire) (1.1.7-13). Her beliefs are, no doubt, based on the "holy speeches" Tartuffe is fond of giving, but, luckily, even she comes to see that his words are hollow.