Anotrothe (The Successors, #1)

Shane, Evelyn, Connor, and Valerie. Non-siblings bound together since conception by a connection only they have. They also share an invisible friend.

But not all invisible friends are necessarily imaginary.

Consider Anotrothe. Friend and gentle guardian with powerful psychic abilities, he is invisible to everyone except them until they are twelve, when he appears and becomes a teacher at their school.

One day, he tells them that the connection they share gives them the capacity to learn things...