A Wind in the Door

This book is the direct sequel to A Wrinkle in Time. Main character is Meg. Her little brother is Charles Wallace. He is bullied badly at school for being smart and weird. Their parents are doctors Murray. They meet Progo the Cherubim who Meg must pass a test with. Mr. Jenkins has some clones. Meg needs to discern the true principal, love him, and name him.

We also meet Blajeny the teacher. He is the one who set out the tests. He does not seem to be a main character. The enemy the Echthoi want to X everyone I think that means murder. Meg meets Sporos the faradolae. She need to convince him to join them in healing Charles and fighting the Echthoi. 

There are a number of other characters such as Calvin, who is Meg's boyfriend. There is also Louise the snake, he we find out is also a teacher. In the end, Progo gets X'd by the bad guys. But Charles is saved. The majority of the story was Meg learning to kythe (communicate telepathically) with assorted people. There were also other faradolae to convince and rally.