The series takes place on several different worlds, collectively referred to as the Diadem. Travel between worlds is accomplished through the use of magic portals. The physical distance between worlds is irrelevant. Portal travel is based on the mystical geography of the Diadem, which is divided into five layers. The layers are a little like the layers of an onion and a drawing of them is the logo of the Diadem series and is on every book. At the center is a single world called Jewel, followed by the Inner Circuit, Middle Circuit, Outer Circuit, and Outer Rim. Magic is strongest on Jewel, and progressively weaker the "farther" a world is from the center. In the Outer Rim, magic barely functions at all.
The main characters are three adolescents from worlds on the Outer Rim. Each has extraordinary talent for magic. Due to the weakness of the magic on their home worlds, they do not learn about their abilities (or their mysterious destiny) until the events of the first book.
The son of Bad Tony Caruso, a mob boss, Score spent most of his life in fear of his father and the Bowery gangs. His mother died when he was young, supposedly a victim of his father. She was a gypsy and may have originally come from Ordin, Helaine's world. When his father was arrested and jailed, he was put into state custody, but escaped. Score was brought to Treen by S'Hee, an Orca Bestial. His main magic ability is to change objects from one thing into another. But he also has the abilities of sight, water, and the control over an object's size.
This easy-to-be-around joker is the best at magical fighting and the second best planner (Pixel is the best). He constantly fights with Helaine.
Helaine is a female warrior from a medieval world. On her planet, she was a noble, but still considered "just a girl" and therefore wasn't allowed to live her life as she planned. So she disguised herself as a boy with the alias "Renald." In this guise, she was able to wander the castle and learned to sword-fight like the other castle boys, eventually becoming the best among them. As we first see her, her father has planned to marry her off to Dathan Peveral, the imbecile son of a needed ally. After her guise as Renald was ruined by Oracle, she planned to run away. As she was leaving, the Shadows inhabited a group of soldiers sent by the border lords and she was rescued and brought to Treen by the Rahn, a leopard Bestial. Her main magic ability is the power to foresee trouble. Her powers with the gemstones are levitation, communication, earth, and shape-shifting.
At first, Helaine's annoyance was shown at the fact that both Pixel and Score possessed no non-magical abilities. At times she can be very bossy, arrogant, and uppity.
Pixel comes from a world that is so technologically advanced, most of the population spends every moment in a virtual reality environment. People live in computer controlled houses and are served by robots. When we first come upon Pixel, we see that he has become disillusioned with the virtual world and wants to escape. He leaves his house, walking the streets. He sees that there is nothing different, every house and street is exactly the same. Oracle meets up with him, then shows him a slave labor camp and tells him that these people make the things that he requests. Oracle leaves him, and a pack of wild dogs inhabited by the Shadows attack him. He is saved by Hakar, a Bestial golden eagle, who takes him to Treen. Pixel has the innate ability to solve problems and figure things out. Through jewels, he also has the abilities to control fire and air, finding, and calling.
Pixel has a keen mind and due to his powers, he is an excellent problem solver and planner. He has problems associated with being a hopeless romantic. He has had crushes on Helaine, Destiny, and Jenna.
These are the three powerful magicians who ruled the Diadem before Sarman. Though they ruled jointly, they could not stand each other. They were tyrannical despots who only remained together so they could keep their power.
Destiny was a servant of the Triad who betrayed them to Sarman. She had hoped they would rule the Diadem together as king and queen. But Sarman betrayed her as well, showing her treachery to the Triad. As a punishment, they used her as a test subject for the reincarnation process, sending her to Earth crippled. She used what little power she had to take over Toshiro, an electronics businessman, and used his influence to attempt to get back to where she could be powerful again. She put a curse on Score to steal his powers, weaken him, and draw him to Earth. She pretended to help the three so that she could be free of her "prison," the wheelchair and the weak magic of Earth. When they discovered her secret , she kidnapped Pixel and took him to Zarathan, notorious world of nightmares. She supposedly died when Score, Helaine, and Pixel forced Zarathan, which was actually a giant egg, to hatch.
Oracle is not actually a person, he is a hologram created by a spell. He was first cast by the Triad to help Score, Helaine, and Pixel to get to Treen and then to give them clues. Due to the corruption of the magic, he was cursed to speak only in rhymes and to not give the whole truth. After Sarman and the Triad were defeated, Score, Helaine, and Pixel recast him without the curses.
A very powerful magician who killed the Triad and took over Jewel. Once he ruled the Diadem, though, he found he could not leave Jewel without giving up his power. So he set about his plans. He created a great machine called the Analog to represent the Diadem, with jewels for each world set in a large orb. Bound within the world-jewels were the souls of magic users from these worlds. Score, Helaine, and Pixel were the last three, and when he tried to take them, they reversed the spell, binding Sarman to the Jewel
Jenna is a young healer from Ordin, Helaine's home planet. She has a strong rivalry with Helaine and is a magic-user. In the 8th book, Book of Oceans, she joins Pixel, Score and Helaine to help them with their quest. She is also Pixel's girlfriend, causing much romantic tension between Helaine and Score.
Shanara is a powerful enchantress, and a very good friend of Score, Helaine and Pixel. Although she is not as powerful as them, she does help them out often. In the end of the tenth book, we discover that she was the mother of Traxis, and that she is Score's mother.
Blink, a red panda described by Shanara as fat and lazy, that enhances Shanara's powers as her familiar. Blink can be compared to Helaine, Score, and Pixel's gems, except he is alive. Apparently, Shanara can replace him anytime. In Book of War, Blink worries that Shanara will get a new familiar. Not much is known about Blink and Shanara's history. John Peel himself has not worked it out.
When asked if he could explain more on Shanara and Blink's past, he replied, "Not really, because I haven't actually worked it out. I tend to add to my background as I go along. For example, the way Shanara's hair changed color. Originally, it was just a mistake that I caught, because I'd forgotten the color of her hair in book 2. Then I thought, 'Hey, she's the sorceress of change anyway!' After that, her hair is a different color in every book. As to her past, I'm kind of torn between making her really, really old and she just uses her spells to make herself look young, and making her really beautiful, and she uses her spells just for fun. I haven't decided yet!"