The Guardians of Arcadia (or the Champions of Arcadia) are a group of heroic teenagers chosen to protect the universe from any supernatural apocalyptic threats, alongside their closest allies.
The first team, Team Trollhunters, was formed after Jim Lake Jr. became the first human Trollhunter. Its first members were Jim himself, Toby Domzalski, Blinky, and AAARRRGGHH!!!, until Draal, Claire Nuñez, NotEnrique, and Gnome Chompsky joined as well. Their mission was to eliminate Gunmar and his horde of Gumm-Gumms (and later Morgana) to prevent the occurrence of the apocalyptic Eternal Night. They would eventually join forces with Merlin to end the war. After their foes were vanquished, Jim, Claire, Merlin and Blinky left the team for New Jersey while Toby and AAARRRGGHH!!! stayed behind as the remaining members and sole protectors of Arcadia, or so they thought.
The second team, House Tarron, was formed parallel to the first team, after the royal heirs to the throne of Akiridion-5, Aja and Krel Tarron, their pet, Luug, and their guardian, Varvatos Vex, crash-landed on Earth in order to flee from General Morando. Their second guardian, Zadra, followed soon after. House Tarron allied with Team Trollhunters and Team Creepslayerz in order to defeat General Morando and reclaim their kingdom, as well as prevent Morando from getting his greedy hands on Gaylen's Core to make himself an unstoppable god and destroy the entire universe. After his defeat, Aja and Varvatos return to Akiridion-5 while Krel remains on Earth.
Team Creepslayerz was formed when Steve Palchuk, growing increasingly paranoid about Jim and his friends, approached Eli Pepperjack and asked him to help him investigate their strange behavior. Eli warily agrees, and Team Creepslayerz was formed. They find out that Jim was, in fact, trying to protect Arcadia and that trolls do exist. They also assist Jim and his friends in helping the trolls to flee after Trollmarket was taken over by Gunmar. Later, they become close allies of House Tarron and help them defeat General Morando and his bounty hunters. After Morando's defeat, the Creepslayerz disband when Eli decides to act as Earth's ambassador on Akiridion-5.
All three of these factions soon realize that Merlin has intentionally founded and dubbed them all as "The Guardians of Arcadia", and they join forces with him, his apprentice, Douxie, and his familiar, Archie, when the fate of the worlds is at stake due to the mysterious and dreaded Green Knight and his allegiance with the evil Arcane Order. In order to save the future, the Guardians must travel to the past and restore history. Douxie, Claire, Steve and Jim get sent back and engage in the events that would eventually lead up to the distant origins of their teams. They eventually make it back to the present, yet are faced with more obstacles such as Merlin's death, safeguarding Nari and the Genesis Seals, and saving Jim from the Green Knight's control (who is later discovered to be the resurrected King Arthur). In the end, the Green Knight is destroyed, Jim is freed and turned back to human, but he loses his amulet and the mantle of Trollhunter is broken (although it is implied that he'll become the new wielder of Excalibur soon). The Arcane Order have the Seals, but they still need Nari to activate them, so she and Douxie leave Arcadia to hide from them while the remaining Guardians stay behind.
- Merlin †: Merlin was Douxie's master and the patron of the Trollhunters. He was (technically speaking) the founder and leader of the Guardians, as well as one of the group's top advisors and combatant. However, he is later killed by the Green Knight (who was his former friend, King Arthur himself) and passed down his leadership to his apprentice, Douxie.
- Douxie Casperan: Douxie is Merlin's apprentice, therefore dubbing him a sort of second-in-command of the Guardians. However, after the death of Merlin, Douxie takes up his position as the new leader of the Guardians and Nari's new protector. Ending Arthur's reign of darkness and prejudice once and for all, Douxie leaves Arcadia to protect Nari from the Arcane Order, but promises to his team that he'll return soon.
- Jim Lake Jr.: Jim is the first human (briefly half-troll) Trollhunter, the leader of Team Trollhunters, and the slayer of Gunmar and Bular, therefore making him one of the most important fighting figures of the Guardians. After being freed from the Green Knight's corruption, Jim becomes (technically) the co-leader of the Guardians and thus a leading figure when on the field.
- Claire Nuñez: Claire is Jim's girlfriend and Team Trollhunters' occult expert, as well as Douxie's apprentice in the art of shadow magic.
- Steve Palchuk: Steve is Eli's enemy-turned-best friend, as well as the founder and co-leader of Team Creepslayerz.
- Archie: Archie is Douxie's familiar. His main goal is to guide his master and the other members in his quest to protect the universe and support him in the field.
- Toby Domzalski: Toby is Jim's best friend and the moral support for Team Trollhunters.
- Blinky Galadrigal: Blinky is Jim's mentor and the knowledgeable advisor of Team Trollhunters.
- AAARRRGGHH!!!: AAARRRGGHH!!! is a reformed Gumm-Gumm, Blinky's best friend, Toby's protector, and the field support and muscle for Team Trollhunters.
- Aja Tarron (inactive): Aja is the new regent of House Tarron and queen of Akiridion-5. Although technically considered a member of the Guardians, she is currently unavailable, due to her return to Akiridion-5.
- Krel Tarron: Krel is Aja's younger brother, the second regent of House Tarron, and the king-in-waiting of Akiridion-5. He is considered to be the tech support and one of the most intelligent members of the Guardians.
- Varvatos Vex (inactive): Varvatos is the commander of the Taylon Phalanx in Akiridion-5 and House Tarron's protector. Although technically considered a member of the Guardians, he is currently unavailable, due to his return to Akiridion-5.
- Eli Pepperjack (inactive): Eli is Steve's victim-turned-best friend and his partner in Team Creepslayerz, as well as Arcadia's supernatural conspiracy theorist. Although technically considered a member of the Guardians, he is currently unavailable, due to visiting Akiridion-5 as its Earth ambassador.