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 his 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. He is just as heartless as his boss saying that he has an absolute skill for encouraging contributions. Nonetheless, he does have some degree of morality, as evidenced by his horror that Prince John was willing to have Friar Tuck, a man of the church, be executed (though mostly because such an act against clergy at the time would warrant retribution from the church), even if it was to lure Robin Hood out of hiding. A running gag throughout the movie is Hiss hissing in Prince John's ear.
Sir Hiss is somewhat pompous, as seen when with his interactions with Little John when the latter was taking a seat in the royal box with Prince John. Hiss also seems to dislike being called names (and took particular offense when Little John called him "Buster").
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, after briefly reminding Prince John about his mom (causing him distress), he briefly attempts to hypnotize Prince John to help relax him, but John catches on and tells him not to do that. They are then 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. Nonetheless, Hiss does attempt to prevent John's money from being stolen by Hood, swatting his hand. By the time Hiss escapes from the basket, Prince John has been robbed of everything, including most of his clothing. Sir Hiss begins to lecture the prince and is hit with a mirror for his efforts, despite warning the prince that breaking it would mean seven years of bad luck. 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 in 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. Unfortunately, Friar Tuck and Alan-A-Dale have been tracking Hiss, and before he can report his findings, they shoot him down and stuff him in an ale barrel. 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 evidently had already found out by the time he told him), daring him to get himself out of it if he can.
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 imprisoned for defaulting on taxes, with the news of Friar Tuck's arrest. Though Prince John initially snaps in fury immediately after hearing that, bellowing that he would only feel better if he heard that Robin Hood had been arrested, 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 because the Prince is sinking so deep to catch Robin Hood that he is intending to hang a member of the Catholic Church to do so, which carries the risk of alienating themselves from Rome and the Pope if they learned of it.
During the jailbreak attempt by Robin Hood and Little John the night before the Friar is to be hanged, Hiss is shown sleeping in Prince John's bedroom. He briefly wakes after being kicked by the prince's foot, but is lulled back to sleep by Robin Hood. But later the sound of Prince John sucking his thumb awakens him, which allows Hiss to discover Robin escaping with the gold, 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 outlaw's survival after John thought Robin had drowned. Hiss then goes into a lecture about how Prince John's traps never work and ends by pointing out the damage to Prince John's mother's castle, as it had been set ablaze earlier by the Sheriff trying to catch Robin Hood. This makes Prince John snap, and in a fit of madness, chases Hiss all through the castle, trying to hit him with a wooden stick and calling him derogatory names, while Hiss screams for help, shouting that Prince John had "gone stark-raving mad".
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. He laughs when Prince John accidentally gets his foot crushed with a rock.
- Sir Hiss was one of only two major characters original to the Disney adaptation (the other being Lady Kluck).
- Sir Hiss draws inspiration from Kaa from The Jungle Book, not only in physical appearance, but most notably in the use of hypnosis.
- Sir Hiss' eyes are smaller than Kaa's.
- Unlike Kaa and real snakes, Sir Hiss has flat human-like teeth and he has a gap in his front teeth, a trademark feature of his voice actor Terry-Thomas.
- Sir Hiss is shown to be smarter than Prince John, but the evil prince repeatedly calls him an idiot.
- Sir Hiss is not as evil as Prince John or the Sheriff since he is shown to be shocked when Prince John tells him of his plans of killing Friar Tuck to trap Robin Hood. This implies that he's not really evil; he's merely following an oath he made to Prince John.
- Sir Hiss' reaction has historical basis: the Church during the 12th Century had influence over the Crown at the time, and for a king to punish and execute clergy on their own authority would lead to excommunication or the possibility of inciting other Catholic nations to war, justifying Sir Hiss' fears.
- Sir Hiss wears a green stocking hat to bed.
- Alex Rocco was considered to voice Sir Hiss.
- When declaring that John has gone "Stark-raving mad", he says the last word with an agonized, drawn out scream, implying alongside the pan out to the burning castle that Hiss may have caught on fire, although his next appearance at the rock quarry confirmed his survival.
- In Disney Tsum Tsum, Sir Hiss appears as a non-playable Tsum, and was fought as a boss with Prince John (another non-playable Tsum) during the Disney Storybooks Classic Series II event.