The Hidden Witch by Molly Knox Ostertag

Aster's parents have finally agreed that he can learn witchery with the girls, but he has a lot of catching up to do. So when Grandmother offers her tutoring in exchange for a favor, Aster jumps at the chance, until she asks him to help his great-uncle, Mikasi. Family or no, Aster isn't ready to face the beast who nearly destroyed him and his cousins.

Across town, dark magic has attached itself to Aster's friend Charlie. Witches are taught never to create a "Fetch," a cruel shadow form that does only harm. But the thing following Charlie is a clear sign that someone is breaking the rules.
With the help of his family, Aster does everything he can to protect Charlie. But to discover who's making the Fetch, and to put a stop to it, he'll need darker, more powerful magic, from the most dangerous witch he knows...

Published by Scholastic