In Sakurasou no Pet na Kanojo ending credits Maid-chan's voice actor name is "? ? ?". Why is that?
(I'm Japanese, so please excuse my English.)
Maid-chan's voice actor is Horie Yui, the same one as the Ryuunosuke.
This is the last episode's ending credit:
She has a radio program called 堀江由衣の天使のたまご (Horie Yui's Angel Egg) where she talks about the "???" and how the anime staff asked her to keep it a secret in the the broadcast that aired on 2013/4/7 in Japan.
From the unofficial text transcript of the broadcast.
I will reprint and translate only conclusion.
As it turns out, I even, do not know.
This is the official conclusion.