Magical Girl Mechanical Heart Novel

Luna has always wanted to be an Earth Guardian, but she knows it's far too late to have been chosen. It's a fool's dream anyway. The reality of battling monsters every few days to protect the human race is somewhat less 'Saturday morning cartoon' and somewhat more 'child soldier war story.' But that allure of magic always called to her... and one day, she's given an offer to have everything she's ever dreamed of. She promptly says no, because the offer is from one of the most obviously evil women Luna has ever seen in her life. She's pining, not stupid. Get out of here with that blatant monkey's paw crap. Unfortunately, it turns out obviously evil women don't take to being told 'no' all that well. The next thing Luna knows, she's waking up in a robotic body, unable to speak and bound to follow the whims of the monster she tried to reject. How can she find freedom when her every thought is bound in servitude? Will any of the Earth Guardians she's now being forced to fight realize she's in need of help, or are all of them far too busy with their own problems—including but very much not limited to saving the world? And if she does get saved... is there any chance she could keep the robot body?