Earlier versions of the storyline, drafted in 1943 during World War II, had the two cats appear as a sinister pair, in reference to the then-current war with Japan. Originally named Nip and Tuck, they came to the house with Aunt Sarah while Jim Dear and Darling were still at home and made Lady look bad by committing a series of crimes such as stealing the baby's milk and robbing the icebox, then pinning them on her. In the novelization of the film by Ward Greene, written during its production, they tearfully expressed remorse over causing Tramp's impending execution by hiding the rat's body as a joke, and then tried to make amends.
Si and Am are a sinister pair of cats who use the love they gain from Aunt Sarah to get others into a large amount of trouble. The cats are sneaky and love to cause trouble and make messes for their own amusement. It is indicated that they care for nothing but each other and take advantage of Aunt Sarah, seeing her as a protector and not as the beloved owner she is.
Si and Am are identical seal-point Siamese cats. They are sleek and elegant cream-colored felines with angular, wedge-shaped faces, and large, blue almond eyes, which are oval-shaped and narrow out to points near the ends. They have a deep brown facial point with a darker, almost black muzzle, and dark triangular ears. Their noses are black and small, and they both have two middle incisors that hang over their lower lip; these buck teeth are a commonly used Asian stereotype during the late 1940s and 1950s. Si and Am also have dark brown "stockings" on their front legs and shorter "socks" on their hind legs and have dark, long, winding tails.
In the film, Si and Am's owner Aunt Sarah is in charge of Lady and the newborn child. The two cats are commonly seen toted in a picnic basket owned by Aunt Sarah, from which they emerge after her brief departure.
While Aunt Sarah is upstairs, Si and Am perform their trademark musical number, "The Siamese Cat Song". During their song, they wreak havoc throughout the household, claiming it as their own, and even attempt to eat both the family fish and family bird. During the closure of their song, Lady ferociously chases after them upon hearing of their intentions to sample some of the newborn baby's milk. Collateral damage ensues, and although the cats caused all the trouble, Si and Am feign injury in order to get away with it and lay all the blame on Lady. Si and Am are carried upstairs by Aunt Sarah as they deviously shake their tails. As a result, Lady is muzzled by Aunt Sarah. They are not seen for the rest of the film.
Si and Am reappear in the sequel to the film. They are first seen in the opening of the film while Jim Dear and Darling are taking a stroll in town, where they meet up with Aunt Sarah. She claims she will be attending their family picnic, as Si and Am love Independence Day and are seen hissing back at Scamp. They later appear in Scamp's fantasy as he wishes to be a brave enough dog to take on threats like Si and Am. Finally, they attend a picnic with their owner. During the picnic, they are seen attempting to steal the turkey until Scamp arrives and scares them into Aunt Sarah's arms. They are not seen for the remainder of the film.
They make a notable appearance in Mickey's House of Villains during the villains' hostile takeover of the club. They are seen singing a part in "It's Our House Now!", and later on witnessing Mickey Mouse's climactic battle with Jafar. In the end, they flee with the other villains, after Jafar is sucked back into the lamp and defeated.
They appear in the 2019 live action remake. Due to the cultural depiction considered inappropriate in the original, their breed is changed from Siamese cats to Rex cats and their names have been changed to Devon and Rex (voiced by Nate "Rocket" Wonder and Roman GianArthur, respectively). "The Siamese Cat Song" (which uses a Southeast Asian melody) is replaced with a jazz number called "What a Shame", sung in a jazzy melody.
Much like in the original, they are Aunt Sarah's cats and come out to destroy the house (albeit, more destructive) while blaming Lady for the damages. However, when Lady returns to the house with the Dears, Sarah is kicked out along with her cats who can be heard meowing in agony as they are forced to leave as comeuppance.
In Fantasmic!, Si and Am appear in the floating bubble musical sequence.
- These two characters have been cited as Asian stereotypes because of their exaggerated accents, yellow fur, squinty eyes and bucked teeth (or in their cases, fangs). These features have been altered in the sequel and House of Villains. The original film has not been altered, but on Disney+ it carries a disclaimer warning of potentially offensive content.
- With their names combined, "Siam" is the former name of the Kingdom of Thailand, referencing the cats' heritage and the name of the country.
- It has often been debated whether Si and Am were males or females. However, in the Icelandic dub Si calls Am "sister", which confirms that they are females.
- Devon and Rex are male counterparts to Si and Am, like Jaqueline is a female counterpart for Jaq in the Cinderella remake.
- Thus, unlike Si and Am who are female Siamese cats, they are male Oriental cats.
- Furthermore, Devon and Rex's names are derived from the breed name "Devon Rex" which originated in England during the late 1950s.
- Because they are from a Thai breed, they speak with an Asian accent.
- Originally, they were to have been animated by Ward Kimball, but Walt Disney decided it was too fluidly animated and as a result, almost all his animation except one or two shots according to Kimball, were cut from the final film.
- They are the only Lady and the Tramp antagonists to be constantly involved in Disney Villain merchandise, without mentioning Aunt Sarah or The Rat.
- The Siamese Twin Gang from Chip 'n Dale Rescue Rangers bear a striking resemblance to Si and Am.
- In this credits to the danish version of the 2019 version of Lady and the Tramp, Devon and Rex are still called Si and Am, for uknown reasons.