"You're not crazy. You should realize that by now... but fine. If you let me help you, and you learn to sing again as you did before, then I will leave you alone." Once he was sure that she wouldn't simply pretend to learn and forget all about it once he was gone, then ceasing to masquerade as this voice might not be a problem. By that time Bran hoped that things between himself and Lotte - even if it was 'Trevor' at the moment - would be different, so he wouldn't need to do this anymore.