Artist's commentary
Hibiki-sensei: Mara
(He's the Lord of Rodly Might after all, no need to add mosaics or anything, yeah, he's the Rodly Lord, yep.)
"The O-Most-Splendid-One" in the original, one of the possible euphemisms for you-know-what. Along with "Mara", which is why SMT Mara is also nicknamed "The O-Most-Splendid-One" in the Japanese fandom.