When Meg, Calvin, and Mr. Murry arrive on on Ixchel after escaping IT, Meg is paralyzed and mostly frozen. She is put in the care of one of the planet's inhabitants, a frightening looking yet extremely gentle creature who holds, comforts, warms, bathes, and feeds Meg until she recovers.
Meg quickly grows to love this creature, but she cannot think what to call her. As Meg thinks about it, the creature gently examines the girl's thoughts, and finally suggests that Meg call her “Aunt Beast.” Meg laughs a little but agrees that this is the perfect name.
Aunt Beast makes several other suggestions (some of which are actually gentle commands) to Meg while she has the girl in her care. She tells Meg, for instance, that she needs rest and food, quiet and peace, so that she can fully recover from the effects of the Black Thing.
When Meg becomes inordinately angry at her father, Aunt Beast tells her fellow Ixchel inhabits, as well as Mr. Murry and Calvin, that the spiritual effects caused by the Black Thing linger after the physical effects. They need to be patient with Meg, and Meg needs to be patient with herself and others. When Meg struggles to describe Mrs. Whatsit or Mrs. Who or Mrs. Which, Aunt Beast suggests that she not use words but rather think about what these three beings are. Meg still struggles though, because she doesn't know exactly what they are. Aunt Beast starts to grasp something, but then the beings themselves arrive.