Si and Am are two siamese cats in Lady and the Tramp.
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.
Si and Am are sinister (in the spongebob series, just misunderstood and mischivious) 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 amusment. in the Spongebob verson of the film, however, they're just trying get new friends with the wrong methods.
Role in the series
The Siamese Cats will not appear as villains, cause of Scroopfan's wish to see them in a better light. just recently, he came up with a very interesting idea for them, instead of their cannon role, they instead became mischeious side-kicks and the love intrest of Max the cat, who happens to be mostly in to Si.