It's not a romantic, light-hearted, rom-com is it?
I mean, it is horror, right? I've seen it listed as a horror movie.
(The slasher bit I'll admit to maybe being wrong on.)
Well of course it is a horror. Had you called it a horror movie I would not have disagreed, but you didn't, did you.
I'd hesitate to call Pinhead even a principle character in the first film, it's a grim tale of love, lust, greed and betrayal. Pinhead has all of a few minutes of screen time and is not essential to the underlying plot. They didn't adopt any slasher film elements until the 3rd film and did so as a result of sheer money-grabbing laziness. The rest of the films are not even worth mentioning in the same sentence as the first two.
You shouldn't let your dislike of the slasher genre stop you watching the first Hellraiser movie, it's a great film and a classic of the Horror genre as a whole.