The Guardians of Arcadia (or the Champions of Arcadia) are a group of heroic, honorable warriors chosen to protect the universe from any evil supernatural threats, aided by their mentors/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 Claire Nuñez 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 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 no more (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, but he promises to return to reunite with his newfound family soon.
The entire Guardians of Arcadia begins to fully come together in Rise of the Titans, where they are forced to face the Arcane Order together once again. After a failed attempt to destroy the genesis seals, the heroes split up to take one titan down. With two titans down, the heroes must face the strongest and most powerful titan. In order to increase their ranks, Jim is forced to pull out of Excalibur but fails, but with the help of his friends, he successfully pulls Excalibur out, and in new magic/akiridion armor, Jim ultimately kills the final member of the Arcane Order, Bellroc with Excalibur, with the aid of Toby Domzaski, who uses a trifurcate radiation generator to nullify Bellroc from using their magic. However, Toby is gravely wounded after being crushed by debris. After Toby gives his last breath, Jim decides to use the Kronisfere to travel back in time to the morning where he found the amulet, and ultimately allows Toby to take up the mantle of the trollhunter.
- 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 onto 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 wich happens one year later in Rise of the Titans to defeat the Arcane Order once and for all.
- 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 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.
- 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 general, Blinky's best friend, Toby's protector, and the field support and muscle for Team Trollhunters.
- Aja Tarron: Aja is the new regent of House Tarron and the new queen of Akiridion-5.
- Krel Tarron: Krel is Aja's younger brother, the second regent of House Tarron, and the king of Akiridion-5. He is considered to be the tech support and one of the most intelligent members of the Guardians.
- Varvatos Vex: Varvatos is the commander of the Taylon Phalanx in Akiridion-5 and House Tarron's protector.
- Steve Palchuk: Steve is Eli's enemy-turned-best friend, as well as the founder and co-leader of Team Creepslayerz.
- Eli Pepperjack: Eli is Steve's victim-turned-best friend and his partner in Team Creepslayerz, as well as Arcadia's supernatural conspiracy theorist and the first Earth ambassador of Akiridion-5.
- Morgana (briefly) †: Morgana was Merlin's finest student and the sister of King Arthur himself. She was chosen by the Arcane Order to become their champion, the Eldritch Queen. Due to her fall into darkness, she was imprisoned by Merlin and his other apprentice, Douxie, during the Battle of Killahead Bridge. Nine hundred years later, she was freed by Gunmar and proceeded to bring forth the Eternal Night, until she was ultimately defeated at the hands of Team Trollhunters and is banished into the Shadow Realm. Later on, the Arcane Order released her yet again to use her as a pawn, but during her time in the Shadow Realm, it gave her time to reflect on her mistakes and see the error of her ways. Ultimately, she renounced the Order and joined the Guardians to help stop them and the Green Knight. She sacrificed herself to strip her fallen brother of his right to wield Excalibur and squash him with a large debris. Later on, her spirit joins Merlin in the afterlife to make amends with him.
- Gnome Chompsky
- Vendel †
- Draal the Deadly †
- Angor Rot †
- Mary Wang
- Darci Scott
- Shannon Longhannon
- Detective Scott
- Lenora Janeth
- Karl Uhl
- Coach Lawrence
- Stricklander †
- Nomura †
- Barbara Lake
- Tronos Madu †
- Nari †
- Charlemagne (inactive)
- King Arthur/Green Knight †
- Morgana (formerly) †
- Arcane Order †
- Gumm-Gumms †
- Queen Usurna †
- Angor Rot (formerly) †
- General Morando †
- Colonel Kubritz (formerly) †