Gandalf explicitly labels Bilbo "Burglar" and if anyone anyone should know what class someone is it would be him.
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Gandalf is also known to have a similar attitude towards truth as Obi-Wan or Yoda. He has "read the portents" and decided that the Dwarrow never do wells and misfits NEED Bilbo, regardless of their views on the matter.
But I'd make Bilbo a thief rogue as well, because Bard in DnD is connected with magic and as far as I remember, Bilbo was not capable of it. Yes, he had some magical/rare artefacts in his disposal, but thats it.
Yes and no - we are trying to shoehorn a character from one "system" into another - the fit need not be perfect. A rogue with bardish overtones, or bard with rogue skills - either will fit. As I regard a Thief to be more "in your face" (or rather - in the back) than a Bard hence my leaning towards a Bard. But see previous sentence.
BTW - Bilbo was scholarly - College of Lore it is, then
That gives him Epertise in Sleigh of Hand and Deception ...
