"They could be repelled, with a spell called the Patronus Charm, which is made of happy memories. Most people's looks like an animal made of silver light. Miranda's looks like a person, and instead of shooing Dementors away it chases them down and kills them. She saved our lives - we were looking for Jenny, me and her and Emma and Professor Fisher - and there was a Dementor that had Jenny cornered, and she'd morphed her mouth away so it couldn't take her soul, but she couldn't run away. Professor Fisher fainted. I couldn't do anything, Emma couldn't, but Miranda knew the spell and she'd never gotten it to work before but that time it worked and the Dementor was gone."