Brimston straddles the line between bat and demon. While he was not born of the deepest fiery depths (else he may have been named Brimstone with an e), he still has some nasty personality traits.
He constantly produces a slime - from his nose, eyes, and mouth - that inevitably coats everything he comes in contact with. This includes his trusty staff from which he displays the day's trophies. The slime is like a pheromone-signature: one sniff and other creatures know he owns a particular item (or has at least made a claim on something).
Brimston spends his day collecting: shiny objects; squishy objects; smelly objects; foul objects. Generally speaking, if it's shiny or gross, Brimston must have it. And if it's both? Even better.
He prefers to travel at night, but has been known to venture out in the daylight when he's on the trail of a particularly coveted target.