i wrote this a lil' back to answer this very question!
i have an odd username story.
a long time ago, around 1st–2nd grade, i was playing mc at a friend's house. their parents did computer-y work (which involved computers,) so there were 2 pc's: enough for 2 people. when joining through the computer, you could choose your name, and somehow, i picked "Adphee".
that, however, would be irrelevant, as my first real username was "dErPy_KiRbY", after my derpy attempts to draw kirby ;-; i later switched to "dErP_kErB" (the case symmetry is simply more pleasing.) i used it for a long while, but eventually, i found it was taken on certain platforms (namely gmail.)
meanwhile, "Adphee" had maintained some underground usage: as my password (i deleted the email tho, so good luck hacking into stuff ;-) since no website allows for 6 character passwords, i had to make some adjustments, which led to the addition of "XD". like the root-name, i have no idea where "XD" comes from besides sounding cool, but it sorta stuck.
because i hated compromising my username on gmail, i rebranded as "AdpheeXD"; shortly after, i stumbled across splatoon 3. 2 had fascinated me enough a while back, but all the hype over 3 had me hooked. i certainly wouldn't game as "Player", so i had to pick a name.
now im not your average person, willing to accept a normal, legible name. no, i needed S¥ʍßøLs. every single letter of my name (barring H and d) became a symbol [note: H is capitalized because it led to pH, which i thought was cool.] i even changed "XD" to "Ω²".
[side note: although i simplified my ign, i always kept the "Λ". i love that specific character because of mariokart wii, which uses it over a standard capital A. i remember discussing it with my mom, and at the end of the conversation, i was left thinking "that's a really cool thing." anywho...]
having all the symbols was a bit much :/ yet i couldnt let go of one specific character: "æ". this, along with my newly needed discord reset (like i said, i deleted the old email,) led to some experimentation. thus, æ became part of my official name. ironically, æ changes the pronunciation. most people read "Adphæe" as "add+fae", despite æ being pronounced as "ee". nonetheless, the pronounciation stuck, so now im Adphæe. my username has become such a big part of me: so much so i mentally call myself it over my irl name XD
so... that's me ig