dA as in Deviant Artist? I'm one ( sort of)!:D
Or did you mean Devil's Advocate?
Desu has to do with an anime called Rozen Maiden, the most popular character among fans is a doll that always says 'desu' at the end of every sentence, I think it means 'it is' or something, not sure...... Consequently, the char herself is normally referred to as Desu. She's very popular at 4chan, which all those users are likely to be member of, or have at least been there, or something......