Here's what I put together this summer, as I was rereading all of the JC appearances. I only got up through Swamp Thing 76 and Hellblazer 8, so the entries are only up to date through JC:Hellblazer 8.
Bella Donna [JC 3] or just Donna to her victims. She is one of the "yuppies from hell." Her favorite earthly wine bar is "the Pits".
Blathoxi [JC 3] The demon lord of flatulence. Servant to Mammon. Apparently bested by JC before the events of JC 3. Tricked again by JC in that same issue, when John offers to sell his soul to Blathoxi.
Rodney Bubosganglia [JC 3] One of the "yuppies from hell". His partner is Bella Donna. He uses the name "Rodney" when in cognito.
The Brujeria [ST 46] A secret society of male witches based in the forests of Pata(s/g)onia, at the southernmost tip of South America. They created the Invunche and hoped to benefit from the psychic after- effects of the Crisis.
"Chas" [JC 1] Cab Driver and one of JC's only old accquaintences who doesn't end up dead. Chas has some links to illegal activities, but is trying to stay clean. His wife detests JC, His sister is a prostitute.
Benjamin Cox [ST 37] "Spooky twelve year-old genius" (at Newcastle) - He provided JC with a copy of the Grimorium Verum, which was used to summon Nergal (by mistake) at the Casanova Club. After Newcastle he has a pronounced stutter, and returned to Wisconsin to live with his mother. The two of them are killed by Judith some time between ST 46 and ST 48. Ben reappears as a member of the "Spook Group".
Emma [ST 37] American artist living in a Greenwich Village loft. She is the first victim of the Invunche (ST 37) as it chases her through a window. She's the first of the "Spook Group" to start bothering JC. (JC 1)
Funky Flashman [ST 76] last seen trying to set the Swamp Thing up against Darkseid.
Eddie Flynn [JC 4] London bookie who has learned not to take JC's bets. He has almost learned not to take Chas' bets.
Satchmo Hawkins [JC 1] A reporter (from XS) who never got the exclusive interview with JC that he wanted.
Robert "Piggy" Huntoon [ST 66] An old schoolmate of JC's. They had a falling out over a woman named "Diane". He is the author of the book "Pow! Psychology: Understanding the Super-Men (and Women)".
Roger Huntoon [JC 7] JC's supervising doctor at the Ravenscar Secure Hospital. His applications of electric shock and drugs didn't improve JC's temperment.
Invunche, The [ST 37, secret origin in ST 46] "To make an Invunche, they (the Brujeria) take a baby about six months old, and then they disjoint the arms, and the legs, and the hands, and then they..." The Invunche was responsible for killing a number of JCs friends: Emma, Sister Anne-Marie, Benjamin Cox.
Lyndon Johnson [JC 1] JC had hustled a number of bookies by predicting the hour of the former American president's fatal heart attack. Consequently there are very few London bookies who will accept any of JC's bets.
Judith [ST 37] She met JC while studying Tantric Yoga at the North Beach Ashram. After Newcastle, she slipped into the punk scene, eventually resorting to drugs to keep her from sleeping (and being tormented by dreams of the Invunche.) She later joins the Brujeria and offs Ben Cox, his mother, and Frank North. In ST 48 she is turned into a bird, and used by the Brujeria to summon the Dark. (Don't bother asking what type of bird, Cain thinks it was a crow, Abel thinks it was a raven....)
Gary "Gaz" Lester [JC 1] Before Newcastle, "Musician, counter-culture clone, and small time conjurer." A founding member of "Mucous Membrane". Later a heroin junkie who releases Mnemoth, the hunger demon. He is eventually used as bait to trap Mnemoth, which consumes him. As a ghost, he joins the "spook group."
Mammon [JC 3] The Arch-demon of Profit. The mastermind behind the "yuppies from hell" scandal. Mammon investments contracts for "all intangibles in perpetuity."
Cheryl Constantine Masters [JC 4] JC's older sister.
Gemma Masters [JC 4] JC's only niece. Loves her uncle, hates her father.
Tony Masters [JC 4] JC's all but useless (to hear JC tell it) brother- in-law. At one time, a dupe of the Resurrection Crusade.
Papa "Doc" Midnite [JC 1] Haitian Voodoo Master based in NYC. Also runs a variety of illegal operations. He considers himself a "Hougan", and he is probably right.
Mnemoth [JC 1] A hunger demon that was originally bound in the body of a small boy, to protect his village from famine. Mnemoth was released by Gary Lester and placed it within a bottle, which he forwarded to NYC, thinking it would reach JC. Eventually captured and destroyed by JC and Papa Midnite.
Ray Monde [JC 3] An old queen who runs a bric-a-brac shop, "Serendipity". Ray has a "feel for the bizarre". It was on Ray's insistence that JC investigated the Casanova Club in Newcastle. Ray also clued him in about the "yuppies from hell." Ray had AIDS, but ended up being beaten to death by members of the Resurrection Crusade, taking his rightful place as a member of the "spook group."
Mrs. McGuire [JC 1] JC's landlady. She buys the farm in JC 10, courtesy of Nergal. (as does Mighty Mouse, a rasta who also rents from Mrs. M)
Nergal [JC 6] Originally summoned to this mortal coil by JC at Newcastle, the demon could not be held because JC didn't know his true name. Nergal was the driving force behind the Damnation Army, and it is his ichor that flows through JC's veins.
Frank North [ST 38] A biker/anarchist based in California. He has some familiarity with guns and explosives (as seen at Newcastle). He is killed by Judith in ST 48. In JC 2 he takes up haunting Constantine along with other members of the "spook group".
Ritchie Simpson [JC 7] Computer Freak, Deadhead, and Quantum Magic pioneer. A member of the Newcastle team, his body spontaneously combusted while his mind was busy floating through a computer network.
The Tea Party at Baron Winters [ST 50] consisted of Steve "Mento" Dayton, Sargon the Sorcerer, Dr. Occult (who was uninvited), Zatarra, Zatanna, Baron Winter and JC. Casualties include: Sargon, Zatara, and Mento's sanity.
Zed (Mary) [JC 4] Mystery woman. It's better if you just read the book.
With all the Bruhahaha about Zatanna I suppose I should have put her on this list too...
-- + Mike Kelly, Notre Dame Department of Physics +