Chocky - John Wyndham

Matthew, they thought, was just going through a phase of talking to himself. And, like many parents, they waited for him to get over it, but it started to get worse. Matthew’s conversations with himself grew more and more intense—it was like listening to one end of a telephone conversation while someone argued, cajoled and reasoned with another person you couldn’t hear.

Then Matthew started doing things he couldn’t do before, like counting in binary-code mathematics. So he told them about Chocky—the person who lived in his head.