Harrow to Callum, oddly enough, and in one particular instance. Harrow gives in to his hatred of Avizandum, goes along with Viren's Dark Magic-inspired plan, and seeks the dragon out to kill him. Callum acknowledges that he hates the fallen dragon...but then expresses deep remorse that Zym's father was taken away from him. In part, this foil happened because Harrow learned from his mistake, and imparted the wisdom of forgiveness and love to his stepson.
Claudia to Callum. Both are mages, and tempted to use Dark Magic to save the people they care about. But whereas Callum decides to reject Dark Magic for good after using it only once, Claudia jumps off the deep end, using a living creature for Dark Magic and never questioning the morality of her actions. Callum further develops into one for Claudia after the Time Skip. While both have grown into skilled mages and can be pushed to extreme actions on behalf of their loved ones, Claudia continues to resort to Dark Magic without hesitation, while Callum will only ever used it when he has no choice. Furthermore, Callum is selfless in his motivations, using the power for the sake of others even at great personal cost, but Claudia will even go against her family's wishes to save them, because she doesn't want them to leave her. They also have differing dynamics with their siblings. Claudia is the younger child who refused to go with her older brother when he switched sides. Callum on the other hand is the older child, and has remained devoted to his little brother throughout the show.
Viren to Callum. In addition to both being mages with a decent amount of insecurity about their place in the world, both have drastically different approaches to magic. For Viren, it's purely a tool to increase his own power, both to implement "creative" (read: expedient) solutions to his problems while also to manipulate and control the people around him. For Callum, at first he seeks magic to feel useful, but once he has it, he uses it as a means to empower, defend, and protect his loved ones, rather than to benefit himself. Notably, he would use magic to help Rayla find closure and a sense of agency, or grow wings to save her life. Viren never thinks to use magic in this way. The most striking difference between them comes in regard to their willingness to sacrifice themselves for others. Viren hesitates (and then declines) to lay down his life for King Harrow. Callum, on the other hand, does not hesitate to attempt to sacrifice his life for Ezran (by pretending to be him when Rayla is trying to kill him).
Finnegrin to Callum, both are magic users who lost individuals close to them to archdragons, with both wishing to gain more power. However, where as Callum is caring towards his friends and wants to help others, Finnegrin is a completly selfish Bad Boss who abuses his crew and allies just to prove his superiority. Callum lets go of his hatred towards Avizandum in favor of bringing peace to the world, while Finnegrin never lets go of his hatred towards Domina Profundis and wants to rule the ocean. Lastly, Callum learns that he can't have control over everything, including magic, while Finnegrin belives that he has power over everyone and everything.
Naturally, Rayla to Callum. On the surface, Rayla appears to be the kind of person that Callum is either expected to be (excellent swordfighter, good at horseback riding, great strategist) or wants to be (imbued with magic, possessing broad knowledge of magic in Xadia). This is highlighted when Rayla successfully pulls of a "sweep-the-leg" maneuver that Callum tried yet fell short. Despite this, Rayla has deep-seated insecurities that rival Callum's, holding herself to punishing standards and not having an accurate assessment of herself (both qualities bear striking resemblance to Callum). Rayla and Callum process their vulnerabilities very differently; Rayla buries and hides her emotions and insecurities from others and presses on, while Callum openly declares he can't do anything even when others point out that this isn't true and gives up quite easily. Rayla and Callum also resolve these issues in different ways too, Rayla by realizing that bottling up her feelings is actually her chief weakness, while Callum realizing that his affinity for art (which he dismissed to the point that he doesn't even consider it an important skill) is his chief strength.
Foils to Viren
Amaya to Viren. Both are extremely close to Harrow, but where as Viren is a deceptive mage that delves into dark magic and has trouble relating to the princes, Amaya is a Cool Aunt to the princes, but has been away for a long time, due to being in the frontline during the war. Viren is also a mage and subtle manipulator who favors trickery and subterfuge, while Amaya is a warrior who takes direct approaches and is never subtle about anything.
Aaravos to Viren. Both are The Archmage for their respect regions, but Viren specializes in dark magic while Aaravos has mastered all six primal sources. While Viren looks relatively old and is capable of losing is cool pretty easily, Aaravos has fairly youthful looks and is always looking cold and calm and besides that he only ever shows surprise and smugness.
Foils to Janai
Amaya to Janai, her opposite number among the Sun Elves. Janai also is the sister of her country's queen, whom she loses due to Viren, is an honorable warrior, and is an example of Rank Scales with Asskicking. Janai essentially what Amaya would be if she were an elf. Where Amaya wears blue and silver and favors using a huge shield, Janai wears red and gold and favors a big sword. The two are both honorable warriors and have a shared trauma of losing their older Queen sisters due to Viren, and end up bonding over it. It's not really surprising the two become friends over the course of the third season.
Karim to Janai. Both lost their sister Khessa to Aaravos and had to abandon the city they grew up in. However, whereas Janai chose to help humans and supported the idea of peace between the Human Kingdoms and Xadia, Karim came to blame humans for everything bad that happened to him and refused to change his ways no matter how much Janai tried to reason with him.
Kazi to Janai, mainly for the audience's sake. They quickly show that not all sun elves are imposing, stoic warriors like the ones we've seen so far.
