Since childhood, King Trevor and King Frederic have been rivals in pranking and outdoing each other with King Trevor always looking to humiliate King Frederic and prove Equis is the superior kingdom compared to Corona. Despite the constant humiliation by King Trevor, King Frederic refused to participate or enable his ongoing shenanigans. Aside from their prank wars with one another, King Trevor and King Frederic's rivalry was further increased when they both fell in love with Queen Arianna and competed against each other to win her heart until she chose and married King Frederic. Ever since, King Trevor has remained obsessed for years to the point where he has his face covered over Frederic's in a portrait of the royal couple, but is forced to respect their marriage due to Inter-Kingdom law. Eugene even comments that Trevor still has a heart, as proven by his immense love for Trevor Jr. and the returning of Herz Der Sonne’s diary (which Trevor stole). His rare kindness was later shown when he finally accepted Arianna and Frederic’s love for each other after seeing how they were willing to die with each other. Nevertheless, he is still very cruel.
King Trevor also has a strong love for sea creatures, particularly his pet seal named Trevor Jr. which he often babies and is overprotective, but is oblivious that he is the source of Trevor Jr's unhappiness. Although he is a king, his attitude is that of a vengeful and spoiled child who only cares for his own well-being and is not afraid to go to great lengths to ensure he gets what he wants. King Trevor also values high security in his kingdom as he has over a thousand guards on duty and boobytraps within his castle, making it impenetrable. However, despite King Trevor's various Guards and high security, Flynn Rider is the only thief who has ever managed to successfully infiltrate his castle's defences.
Role in the series
In "In Like Flynn", King Trevor sends his agents to Corona to deface King Frederic's statue with graffiti and a sign reading, "Equis Rules". Finally fed up with the never-ending humiliation from Trevor, Frederic plots with Eugene Fitzherbert to steal the royal seal of Equis to humiliate Trevor, but they are captured by the guards.
After a conversation between Eugene and Frederic, Trevor walks in the jail room with his old seal Trevor Jr. Frederic demands Trevor to release Eugene and himself, but Trevor tells them he intends to do so only after parading his rival in nothing but a jester's hat in order to show that Equis is "superior" to Corona. He then leaves the duo to wallow in the stench of their own defeat.
After King Frederic and Eugene escape from jail, they run into King Trevor and improvise by tricking him into stumbling through his own library's trapdoor. At Eugene's suggestion, they further humiliate him by leaving King Trevor tied to his own statue with nothing but a jester's hat and a sign reading "Corona Rules" in plain view of his subjects.
In "The King and Queen of Hearts", Trevor learns that King Frederic and his Queen Arianna have lost their memories. Trevor uses this to his advantage to steal King Herz Der Sonne's Journal and to win Queen Arianna's heart. When Frederic and Arianna are on their romantic boat ride, Trevor crashes the moment, causing Frederic (who cannot swim) to be tossed overboard. Trevor rescues Frederic and takes him back into the Corona castle.
While Frederic is recovering, Trevor steals Herz Der Sonne's journal and takes Arianna out into the ocean under the guise of going to watch sea serpents while actually intending to woo her in the middle of the lawless ocean. Rapunzel sets off after them in her ship with Frederic, Eugene, Lance Strongbow, Maximus and Pascal to get Arianna back. When Trevor sees Rapunzel's ship following his, he launches his army of Navy Seals to attack the pursuing ship. Seeing her daughter's ship under attack, Arianna becomes angered by Trevor's demeanor that she punches Trevor in the face before going to Rapunzel's rescue; she also dives into the ocean to rescue Frederic when he falls overboard. This makes King Trevor realize that Arianna and Fredric do love each other. When the sea serpent attacks the group, Trevor rallies his Navy Seals to fight the creature, but it is not until Frederic hands Trevor's sea crystal, which is actually a serpent egg, to the sea serpent that the creature finally leaves. Back at Corona, Trevor bids farewell to the kingdom, allowing Frederic and Arianna to be together, but not before returning Herz Der Sonne's journal, which has become water-logged.