I'm not sure about that, but I'd translate RD? as Are they?, DR! as They are ! Oo R, So DR as Oo ar, So they are (think African-American from the sothern states with an accent as portrayed in films from the 30's, 40's and 50's). The D's could be read as these (dese in the accent I suggested). Characters ... well I'd say there are two people, but I suppose it could mean a letter count as well.
If it's a puzzle question it might be worth reposting in Quizzes and Puzzles with the full instructions. I picked up on it because of recalling that other silly rhyme.