Julian Parker appears to be a reference to John Constantine. His somewhat snarky attitude, habitual smoking, and use of various magics are of a similar vein. In addition to Xombi, he is a frequent ally to Nun of the Above, Catholic Girl, and Rabbi Sinnowitz, although he doesn't seem to fully trust the former. He feels that the Catholic Church is 'responsible for sweeping too much bad stuff under the rug for others to deal with later,' and apparently 'knows a story about a pair of nuns and a stolen handlebar moustache that would curdle your blood.' This distrust appears mutual, as Nun of the Above dislikes the use of magic and believes that embracing the occult as Julian does only invites darkness. Even Rabbi Sinnowitz is shown having some vague suspicions. He wonders out loud how Julian was able to make it to Dakota from Brazil as quickly as he did, receiving only an evil grin and no straight answer. 
Their suspicions may have some grounding, as Julian is shown to have some kind of 'condition' he was in Brazil to seek the cure for that was later shown as a variety of grotesque mouths and eyes manifested on his chest. The full details of this have not been revealed, but even the healing powers of Castle Morgan had no effect. 
Julian seems to know a variety of magic, including an incendiary spell. He also has trapped the spirits of the dead into a bunch of coins, leaving some in various locations he wishes to keep an eye on. These coins can communicate with the coins in his pocket when something goes wrong where they're stationed. He has also shown proficiency in divining, as well as an extensive knowledge of the occult that likely outshines Nun of the Above, Catholic Girl, and Rabbi Sinnowitz. He knows the names of nearly every bizarre creature they encounter, as well as the obscure methods to defeat them. Although he apparently has terrible pronunciation of whatever unrevealed symbol language was required for the spell to seal away the Lord of Fumes.
Out of all of Xombi's allies, Julian seems the most shady. Catholic Girl, and Rabbi Sinnowitz are very friendly and open with the others, while Nun the Less at least is only hostile towards Julian. He has a tendency to withhold information for no explicit reason, and doesn't seem very open to discussing his past. He is also has notably looser morals, while Catholic Girl and Nun of the Above adhere strictly to the teachings of the church, Julian is not above frequent questionable acts such as stealing cars, temprorarily disorienting people who bother him, and said sealing of souls in coins.