User blog comment:B214/Volume 21 info & spoilers/@comment-95.90.224.210-20160602194640/@comment-24.113.252.213-20160604005325

It's think this...

Lucifer: ?

Beelzebub: Ajuka (we know this)

Leviathan: Grayfia

Asmodeus: ?

Mammon: ?

Belphegor: Roygun Belfegor (the 3rd ranker of the Rating Game)

Belial: Diehauser Belial (the Raging Game Champion)

The last two are so obviouse. Hell, it's their own names. Also, Grayfia competed against Serafall for the title of Leviathan, so she's the logical choice.