That reminds me, there was a letter. It's addressed to Yuu...
Miku Hinasaki Yuu's Room Kitchen Counter
Fatal Frame III: The Tormented
The letters from Kei were originally addressed to Yuu. Without being aware of Yuu's tragic death, he still continues to write to him about his investigations and sends him things such as testimony tapes about the Manor of Sleep. Rei uses these materials to help herself in solving the mystery behind the Manor of Sleep.