User blog comment:B214/Volume 22 spoilers/@comment-24593185-20160722022934/@comment-24.113.252.213-20160722025022

You don't have to be a Devil, or a Reincarnated Devil to take part in the IRG. Tiamat is actually a good possibility.

We know that she's working with Ajuka, and she's been the secret judge of the Rating Games for a very long time (though she has only appeared a few times to give a judgement). Ajuka might have had her be Issei's Queen to help him get the hang of being a King. But since she's working with Ajuka, and is the secret judge of the RG, the public might see it as "playing favorites." So she needs to wear a mask to avoid that.