Secrets
Sophie knew Howl still kept secrets from her, but she also know that it was because he loved her. And that's why she wouldn't pry. Because she knew that eventually he would tell her. She kept her own secrets too; some were just hers, some she shared with Markl. The two made a pretty mischievous pair sometimes.
So it caught her off guard one day when Howl whispered into her ear he had a secret to tell her. Normally he would just say the secret, not announcing that it was actually a secret.
Curious now, Sophie followed him up to their bedroom. She had been sleeping in his room for a couple months, and every time she stepped for in it, she resisted the urge to clean the place. There were two things that kept her from doing so, 1) Howl asked her not to, saying that everything was in its place for a reason and 2) she didn't want to find out what the walls were made of. (That was one of the secrets Howl revealed, but Sophie refused to accept his answer as truthful.)
But, right before Howl opened the door, he told her to close her eyes. Slightly worried, Sophie did as she was asked. She heard the door creak open and the felt his hand on the small of her back as he gently led her forward.
She opened her eyes at his command and stared in shock. The room (still messy to a normal person) was cleaned. Things were organized and displayed properly. She could even see the floor.
Again he came close to her and whispered in her ear his secret, he had been afraid to clean this place because something had gotten lose in here years ago and had probably mutated into a monster. All of this he said with a laugh and Sophie wasn't sure if she should believe him or not.
But either way, he was getting better at trusting her with his life, heart, and secrets.