Who is Lizaveta Prokofyena Yepanchin from The Idiot and what is their importance?

Madam Yephanchin is a distant relation to Prince Myshkin. She immediately takes to Myshkin like a son and hates to see the other characters use him. However, she is very eccentric and stubborn and is quick to get angry. Sometimes her disagreements make no sense. For example, she is angry with the Prince for not marrying Aglaya, though she tells him he cannot marry her anyway.