User blog comment:DeathGodPrayer/Talk About What You Want!/@comment-24098614-20140210092545/@comment-1359322-20140210101127

It's not stated, but since the Queen Piece has all traits of other Evil Pieces, so basically it could fit into anyone. Usually, a Devil would choose someone they are closer with and knows well. You can tell by most of the Queens in the Series.

Rias and Akeno, Sona and Tsubaki, Sirzechs and Grayfia, Sairaorg and Kuisha, etc. Usually the Queen is the one closest to the King out of the other Servants. Another thing they also consider is the strength. e.g. Tannin. Though in Tannin's case it's mocre like Mephisto's wry sense of humor. >_>