In order to protect his sister Hisoku Hoshimi (who looks at the stars) to whom the King Azur is really interested, Sage becomes her (Hisoku the sheer of the blue king) and has to sleep with Azur-sama. After making love, two wings appear on Sage's torso, marking him as his.
Hehehehe, for the mark to appear again (to not be considered as a white maiden that everyone can touch), Sage has to go back to Azur, aka the blue king.
5-nin no Ou