Mine is short for Midnight, which in turn is short for SereneMidnight. Midi was just a nickname given by a former friend, and I liked it so much I decided to just go by that.

SereneMidnight, on the other hand... It's partially a reference to a character from obscure JRPG/dating sim Riviera: The Promised Land, named Serene. And her most powerful attacks in that game always start with her name, so Serene Combination, Serene Illusion, or Serene Absolute. The midnight part of the name comes in because I tend to be a night owl on nights I don't have to work the next morning.