King Trevor is the main antagonist of the season 1 episode In Like Flynn, he is the king of Equis and King Frederic's Rival.
Background
When King Trevor was a child, he was always looking to humiliate King Frederic to participate in his joke, but King Frederic said he was not to participate.
King Frederic proposes to 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 let go of Eugene and himself. King Trevor told him to let them go, but not until he would parade him with a buffoon hat showing that Equis was "superior" to Corona and left them.
When King Frederic and Eugene escaped from jail, they formed a plan to humiliate King Trevor, when Trevor told him where the library was he fell into a trap in a silly way for forgetting to install his security system.