Ron James makes his first appearance in "Wizards Only, Fools." He owns a shop in Wizard City called Ron James's Spell Palace. In his store he sells magic items, as well as bottles of many shapes, colors, sizes, and designs, which contain instantly-working spells. While he introduces his shop, he says he has spells for wizards of all levels, with no need for "memorization, incantation, or hard-to-find boolaroo." One simply needs to open the bottle for the spell to work. His shop also has a bottle-returns program.
Later, Finn, Jake, and Princess Bubblegum walk inside to order a cold spell. When Ron James brings one out, Princess Bubblegum is hesitant to buy it. He decides they are not wizards and calls their bluff, pushing a button behind his counter. This calls the police, who break a window and bust in, setting Finn, Jake and Princess Bubblegum on the run. Princess Bubblegum grabs the cold spell before they leave.
Ron James appears to be a good salesperson, as he goes out of his way to appraise his products, demonstrate how the spells work, and convincingly says, "This baby right here... this is what you want, brothers" when showing them the cold spell they needed. He is reluctant to tell Princess Bubblegum what is in the bottle, calling the contents his "secret brew" and refusing to say what it is made out of.
In the episode "Betty," Ron James's lab is "creamed" by Bella Noche's attack on Wizard City, and he is contracted by the Grand Master Wizard to create a "double-negative magic" potion to defeat the creature. Ron James creates a potion using a dirty bidet and a vaporizer, but the result only makes Bella Noche more powerful. The Grand Master Wizard is shocked that Ron James' potion did not work, as Ron James claims to "always deliver," but Ron James claims this was because he had to craft the potion under non-optimal circumstances. Later, Ice King mentions that Ron James told him about Betty Grof defeating Bella Noche.
In "Thanks for the Crabapples, Giuseppe," Ron James accompanies Ice King, Abracadaniel and other wizards on a road trip to Big Butt Rock to try to come up with a new school of magic. He manages to entice some Water Nymphs into accompanying them by using a twinkle-eye potion, and later tries to use an ancient teleportation spell to escape when the bus goes into a swamp, but only winds up exchanging his head with that of Tree Trunks, leaving his head stuck on her couch with Mr. Pig. Presumably, the effects of this wore off later, as the Ancient Sleeping Magi of Life Giving was seen with his head on his own body at the end of the episode after previously trading heads with Ice King.
He is briefly seen in "You Forgot Your Floaties" when Betty Grof is recapping her research into wizards and magic. She watches Ron James being depressed on his couch, but he cheers up after pouring a potion over his head.
Ron James seems to have a rectangular head, but it could instead be a short cylinder or a flattened cube that looks rectangular from the angles seen in the episode. His head is attached to his body with a long neck. His skin is a light shade of minty green. His almond-shaped eyes are set far apart, and his mouth, when open, is oval-shaped. He has a small, pointy but human-like nose. He is shown wearing a long-sleeved light pink shirt.