The Stair Monster is mostly humanoid in shape, but from the waist down where his legs would be, his body tapers down to a tiny, four-wheeled platform. His neck is segmented like an accordion and his head is long and wormlike, with large flappy lips and bulging yellow eyes. As he describes during his portion of singing This is Halloween, his fingers are like snakes and he has spiders in his hair (though only one big spider is shown).
Like most of the citizens of Halloweentown, the Stair Monster is an affable and happy-go-lucky creature who enjoys all things scary and macabre. He enjoys terrifying people, but moreso out of good-natured fun than actual malice.
The Stairs Monster (or the "one hiding under the stairs") is first seen saying the line "I am the one hiding under your stairs, fingers like snakes and spiders in my hair" where he emerges from the stairs. He and the residents of Halloween Town are later seen celebrating Halloween where he and the other citizens welcome Jack Skellington during the opening song "This is Halloween". The Monster is later seen with the other citizens during the Town Meeting where Jack tells the citizens about Christmas where he and the other citizens like Jack's performance. Later when Jack plans to take over Christmas, the Monster and the other citizens are seen preparing Christmas supplies for Jack, which only results in a disaster (as said by the Mayor who found out that the Christmas plan was a bad idea). Later during the end of the film, the Stairs Monster and the other citizens of Halloween Town discover that Jack is alive after he saves Santa from Oogie Boogie and helps him undo Jack's wrongdoings in order of what he did in order to save Christmas as he and the other citizens celebrate Jack's return.