Riddick is written by Edward Wilder.
Julius is quite tall for his age, with long hair dyed black (although he won't admit it), multiple scars, and a tendency to dress rather scruffily. Although painfully thin, he's much stronger than he looks, and is almost never seen without his huge black ax.
Agent Riddick's past is something of a mystery. This may be because he's exceptionally good at keeping secrets from the world at large, or it may be because nobody cares altogether much. He's happy either way.
Julius Riddick hasn't been at the PPC very long, but he likes to act like he's been around for a while. He tries to be aloof, but isn't very good at it, and so he comes off closer to endearingly grumpy.
He knows very little about the Harry Potter continuum, which is why he's been partnered with Leila (that's the excuse the Marquis keeps giving him, anyway). Leila's fangirlishness tends to get on his nerves. He pretends not to be, but he's very fond of his partner and quite loyal to her.