Informal particle borrowed from French 'de', used in casual Japanese to indicate possession, similar to の. Often seen in online handles, nicknames, or playful writing.
from French de
See also: の
I'm Tanaka. (playful self-introduction)
This is mine. (casual, using de instead of の)
Borrowed from French 'de', used as a casual possessive particle in Japanese, likely popularized through online culture and playful language.