As explained in The Diary of a Young Girl, what was the role of "Kitty," or Anne's diary, during her period of hiding in the annexe?

Anne's diary, Kitty, serves as a voice and sounding board for Anne at a time when she was relegated to silence.

Anne's diary, Kitty, allowed her to have a voice.  In both the act of hiding and in her life, Anne lived a life of silence. Literally and symbolically, she was unable to speak. However, Kitty changes that by giving Anne a place where she could speak her mind.  Anne admits this in her opening...

Anne's diary, Kitty, serves as a voice and sounding board for Anne at a time when she was relegated to silence.

Anne's diary, Kitty, allowed her to have a voice.  In both the act of hiding and in her life, Anne lived a life of silence. Literally and symbolically, she was unable to speak. However, Kitty changes that by giving Anne a place where she could speak her mind.  Anne admits this in her opening entry:  "I hope I will be able to confide everything to you, as I have never been able to confide in anyone, and I hope you will be a great source of comfort and support."  In this way, Kitty is a confidant to Anne.  

Anne believes that Kitty provides an opportunity for her to be heard.  Anne feels that Kitty can validate her experience:

Writing in a diary is a really strange experience for someone like me... Oh well, it doesn't matter. I feel like writing, and I have an even greater need to get all kinds of things off my chest.

Anne needs to "get all kinds of things off [her] chest" because she does not have a release.  Kitty's role is to provide this release.  Anne says that she was "prompted to keep a diary in the first place" because she lacks friends. In her role as a friend, Kitty is able to authenticate and validate Anne's voice.