Si and Am

Si and Am are Aunt Sarah's twin Siamese cats and antagonists from Lady and the Tramp. Both were voiced by Peggy Lee. They were later voiced by both Mary Kay Bergman and Tress MacNeille in the sequel; MacNeille now voices them both since Bergman's death.

Development
Earlier versions of the storyline, drafted in 1943 during the war, had the two cats appear as a sinister pair, suggesting the yellow peril. They were originally named Nip and Tuck. In Ward Greene's novelization, they tearfully express remorse over causing the Tramp's impending execution by hiding the rat's body as a joke, and then try to make amends, while in the film they do not partake of the climactic scene.

Personality
Si and Am are sinister pair of cats who use the love they gain 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 takes advantage of Aunt Sarah seeing her as a protector and not a beloved owner.

Lady and the Tramp
In the film, Si and Am's owner Aunt Sarah is in charge of Lady and the newborn child. While Aunt Sarah is upstairs, the two cats cause trouble downstairs. Si and Am torment Lady and wreck the house claiming it as their own. The cats sing The Siamese Song, and during the song, the cats completely destroy the house and even attempt to eat both the family fish and family bird. They even plan to steal the baby's milk until they are stopped by Lady. Although the cats caused all the trouble, Lady gains all the blame and ends up muzzled by Aunt Sarah, which leads to Lady running away and her adventure begins. Si and Am aren't seen for the rest of the film.

Lady and the Tramp II: Scamp's Adventure
Si and Am reappear in the sequel to the film. They are first seen in the opening of the film while the Browns are taking a stroll in town where they meet up with Aunt Sarah as she claims she will be attending their family picnic as Si and Am love Independence Day. They then appear in Scamp's fantasy as he wishes to be a brave enough dog to take on threats like Si and Am.

In the middle of the film, they are last seen attending a picnic with their owner. During the picin, 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.

House of Mouse
Si and Am made numerous appearances in the animated series House of Mouse. In "Rent Day" the cats popped out from there picnic basket cheering for O' Malley and the Alley Cats.

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 climatic battle with Jafar. In the end, the flee with the other villains, after Jafar is sucked into the lamp and defeated.

Disney Parks
Si and Am appear in an extremely large amount of Disney Villains merchandise as well as having a statue of them in the Disney Villains Store in Disney's Hollywood Studios.

Fantasmic!
Si and Am appear in the floating bubble musical sequence. Despite various Disney Villains appearing in the show, they appear without aiding The Queen in her plan to destroy Mickey Mouse. Their roles of not being involved with The Queen are similar to Captain Hook and Governor Ratcliffe.

Trivia

 * While Aunt Sarah is often considered the primary antagonist of Lady and the Tramp, Si and Am went on to become recurring Disney characters, and major members of the Disney Villains franchise.
 * These two character's have been sited as Asian stereotypes because of their squinty eyes and bucked teeth (or in there cases fangs).
 * With their names combined, "Siam" is the former name of the Kingdom of Thailand, referencing the cats' heritage.