User blog comment:B214/Volume 22 spoilers/@comment-189.14.202.69-20161016140306

About Bina LessThan: "Hide your identity with a mask Dragon Only Issei and Ravel knows his true identity.." In reestreia Raiser, he and Ravel fought Diehauser at the time the system of retirement was not working, so Tiamat that acts as a judge intervened in the fight, so Ravel met Tiamat personally. I think Tiamat is Bina, this explains why only Issei and Ravel know her identity. (Excuse the possible errors in writing, I am a fan of the light novel but did not speak english)