Gretel was a Time Weaver. She was at constant odds with her two sisters who would constantly try to rip apart the fabric of the universe, which she would always be there to fix.
When the Twelfth Doctor accidentally landed on one of her sisters, her other sister Ethel tried to get revenge on the Doctor by sending time flies to swarm his time stream and destroy him. Gretel rescued him and teamed up with him to defeat her sister once and for all. (COMIC: A Stitch in Time)