Through my impressive research and investigative abilities, I believe I have discovered the truth behind this mystery. @Melodious Nocturne uses the image of Demyx from Kingdom Hearts. This must have been shortened to DemDem. Also explains who Xiggy is. Xigbar.
The banana is right. I call him “DemDem,” though, because I think it’s cute. But Demyx does canonically call Xigbar “Xiggy,” (although I couldn’t point out to you when, DemDem’s wiki pages do assure that that is the case, as do fan discussions) and it’s rubbed off on a lot of fans. And I am a big XigDem fan myself!
If you look at DemDem’s info page, you can see that he himself gives his name as “Demyx.”
“Melodious Nocturne” should refer to his title in the game. (Most Nobodies have titles given by the antagonist Xemnas. Xiggy’s is “The Freeshooter,” by the way.)
Comments on Profile Post by Nyann