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. However, Eugene 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 neverending humiliation from Trevor, King Frederic plots with Eugene to steal the royal seal of Equis to humiliate King Trevor, but they were captured by the guards.
After a conversation between Eugene and King Frederic, King Trevor came in the jail room with his old seal Trevor Jr, Frederic told him to release Eugene and himself. King Trevor told him he intended to only after parading his rival in nothing but a jester's hat, showing that Equis was "superior" to Corona. He then left the duo to wallow in the stench of their own defeat.
When King Frederic and Eugene escaped 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.
King Trevor returns in "The Brothers Hook", He orders Hook Hand to perform Trevor Jr's destination wedding ceremony. He also appeared in The King and Queen of Hearts to steal King Herz Der Sonne's Journal and to win Queen Arianna's heart.