Like all lions, King Leonidas is arrogant. Even though he is not completely evil, he is sensitive and temperamental. Also, he possesses a deep love for the game of soccer, as seen when the royal championships on his Island could not continue when a suitable referee couldn't be found. He was extremely aggressive and roared at everything that moved.
King Leonidas looks similar to Prince John from Robin Hood, but has a mane, a royal gold crown, and wears a king's red robe.
King Leonidas' voice is quite powerful, which can be expected from a royal lion. His yell to 'stop that ball' is powerful enough to blow everyone away to the net from its hurricane-like force.
He has an anti-human attitude, which may have stemmed from the fairy tale that animals staged a coup on the Isle of Naboombu, killing the original leader, the human wizard Astoroth. He wears the Star of Astoroth, a talisman that is needed to bring inanimate objects to life. After suffering much pummeling and stampeding at a soccer game, Mr. Brown succeeds at grabbing the Star of Astoroth from him. After being alerted by his secretary bird, an enraged Leonidas tries to pursue Mr. Brown, Miss Price, and the children, but he is foiled when he is turned into a rabbit, albeit one with a lion's tail, by Miss Price. He is last seen scurrying away. As Miss Price's rabbit spells are temporary, he presumably returned to his lion form soon after.