Sir Hiss is the snake henchman of Prince John. He often attempts to warn Prince John of the dangers of his various traps, but is rarely listened to. After a failed plan, Sir Hiss often complains about how Prince John didn't listen to him, but he is usually punished as a result. He is often the target of Prince John's temper tantrums, which he usually tries to halt by reminding Prince John about Prince John's mother which never works. Sir Hiss is often compared to Kaa from the film The Jungle Book, as they share similar appearances as well as hypnotic powers.
Sometime before the events of the film, Sir Hiss helped Prince John usurp the throne of England from the rightful king, King Richard. Sir Hiss hypnotized the king into going off on the crusades, allowing Prince John to take the crown in his absence. Sir Hiss then traveled with Prince John from town to town, as the greedy Prince taxed the towns into poverty.
At the time of the film, Sir Hiss is accompanying Prince John to their next stop, the town of Nottingham. During their ride, they are stopped by fortune tellers, who are actually Robin Hood and Little John in disguise. Sir Hiss attempts to warn Prince John that the "fortune tellers" may actually be bandits, but Prince John laughs off the idea of female bandits. After seeing the fortune tellers successfully steal Prince John's rings from under his nose, he once again tries to get the Prince's attention, but the annoyed Prince stuffs Hiss into a basket and sits on it. By the time Hiss escapes from the basket, Prince John has been robbed. Sir Hiss begins to lecture the prince, and is hit with a mirror for his efforts. Hiss is able to get back at the Prince by reminding Prince John that the mirror belonged to his mother.
Sir Hiss's next appearance is at Prince John's archery tournament. Though he initially sits in the royal box, Prince John dismisses him after becoming enamored with "Sir Reginald", actually Little John n disguise. Sir Hiss is secretly tasked with looking for Robin Hood. Sir Hiss hides in a balloon, and by flying about, discovers that a stork archer is actually Robin Hood in disguise. But before he can report his findings, he is trapped in a barrel of ale by Friar Tuck and Alan-a-Dale. He remains in there until after Robin Hood has escaped and Prince John releases him. He belatedly informs Prince John of his findings but is tied to a post as a result (as Prince John evidentially had already found out by the time he told him).
Hiss appears next in a short scene, counting taxes for Prince John. He briefly sings the song mocking Prince John, after hearing of it from The Sheriff of Nottingham, but quickly stops after Prince John comes in the room. Later, he is seen attempting to cheer up an angry Prince John, after most of the town has been imprisons for defaulting on taxes, with the news of Friar Tuck's arrest. Though Prince John initially is enraged, he realizes that the Friar would make good bait for Robin Hood and orders the hanging of the Friar, an order even Hiss is shocked at.
During the jail break attempt by Robin Hood and Little John, Hiss is shown sleeping in Prince John's bedroom. He briefly wakes, but is lulled back to sleep by Robin Hood. But later the sound of falling gold awakens him, and Hiss attempts to grab onto the gold with his mouth. As his tail is wrapped around Prince John's foot, the two are dragged out onto the balcony. Hiss then appears after Robin Hood escapes, pointing out the outlaws survival. Hiss then goes into a lecture about how Prince John's traps never work, and ends by pointing the damage to Prince John's mother's castle. This scares and enrages Prince John, and in a fit of madness, he chases Hiss all through the castle.
The end of the film reveals that King Richard has imprisoned Hiss for his crimes, and that Hiss has been sentenced to work in the royal rock pile alongside Prince John and the Sheriff.