He has many sharp teeth and three black and orange tentacles on his head. During the song This is Halloween he is hiding in the trash can and in the center of town during the festivities that happened after the song. During the preparations for Christmas, he made hats made from dead rats and bats, Making Christmas. He lastly appears at the end of the movie when all of the creatures are confused by the snow.
On the free online building game 'Roblox' a hat entitled 'Halloween Monster' was released. It is essentially the Harlequin Demon's upper head, with the eyes and tentacles. This could be because Roblox and Disney are partners and Roblox will advertise upcoming Disney movies on their website, and at the bottom of the Sponsor page there is a link to Disney's main website. It used to direct users to Disney's other website.
The Harlequin Demon, like the rest of the residents of Halloween Town, loves to scare others and take part in big events and help out in their preparation. Due to being a monster, he has no trouble with frightening and disgusting things and gladly enjoys messing around with dead animals corpses, however he has very little understanding of nice and pretty things due to being a resident of Halloween Town. He also seems to be skilled at turning rotting animal corpses into wearable hats, even considering himself a genius regarding this task.