Buffy began as a television show and only later was adapted into comics. So, it wasn't a comic initially. The TV series gained popularity before the comic adaptations came out.
Buffy's relationship with Spike in the comics varies. Sometimes they have significant interactions and may even be romantically involved, but it's not a constant throughout all the comic series.
The 'Watcher' in 'the watcher horror story' is a shadowy character. We don't have a clear picture of who they are. They seem to have some sort of connection to the house, perhaps from its past. They use these letters to create a sense of fear and unease in the family. Their actions suggest a disturbing fixation on the family's every move within the house. It could be an old neighbor with a grudge, or someone with a more sinister and unknown reason for watching the new occupants.
It varies from comic to comic. Sometimes they have a romantic connection, while in others, their relationship is more complicated or not existent at all.
Buffy does have some science fiction aspects. The show incorporates magic, demons, and other fantastical elements that are common in the genre. However, it also blends in elements of drama and horror.