Why is Maid-chan voice actor listed as '???' in ending credits? 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.

Japanese


  結論から言いますと、私にも、わかりません。


English


  As it turns out, I even, do not know.


This is the official conclusion.