What appears to be going on with ORV Mithra/Mitra/Mithras is that, before the story began, he hired Manu (clearly a lawyer) to represent him in a lawsuit against Christianity, won, and gained control of Jesus’s IP - which includes the username “Master of December 25th” and the “Myth-grade” status.
From a Doylist standpoint, something of this nature is necessary to avoid the implication that Yoo Joonghyuk might have - at some point - killed Jesus.
From a Watsonian standpoint, it’s really funny, because,
1) The Vedic Mitra’s most-famous myth is probably the one where he sees a hot lady and involuntarily ejaculates into a jar. He doesn’t have much going for him. Basically had no choice but to abuse copyright law.
2) Unlike Apollon’s, Surya’s username does not actually have the word “sun” in it. And Manu - who is generally considered to be Surya’s son - uses the words “the sun god” to describe Mithra. Did Manu help Mithra sue his own dad?
3) All of this raises another question: is the 《Vedas》 Nebula’s “Mithra” actually the Vedic Mitra? Or is he the Roman Mithras? Someone else? How many levels of identity theft is this guy on?