As the first thing that one can associate with the word "Bitcoin" is a coin, one should probably go in that direction. Since coins are not very animate in themselves and anthropomorphizing them would probably look silly, probably something that has a strong correlation with a coin is the way to go. One can think of materials - gold and silver, specific faces from the coins (which is too local to consider), and people dealing with coins and money - bankers, tax-collectors, those figures don't really have any good connotations in general. There is one more group of people that are associated with coins - pirates, which whereas can have negative connotations (digital piracy), are also considered positive in some manners (Pirates of the Caribbean, etc.). So I think we should be considering some pirate with a chest full of Bitcoins.
Alternatively, there are also other more fantasy creatures people associate with coins or precious metals - leprechauns, dwarves, and the like. Whereas those don't have much negative connotations, their positive connotations aren't very prevalent either, so I would stick with pirates.