All roleplaying, solo or group is about NPCs. You could play a game with just the player characters and the GM giving vivid descriptions of locations, and throwing monsters at the characters, but it would soon get very boring.
NPCs get to tell the characters about the world, they face problems that need overcoming, they are oppressed or are the oppressors, they are the victims and the villains.
To make horror work you need NPCs that the characters can trust, so you can betray that trust later. You need NPCs that the characters care about, so you can threaten them. You need victims that the characters are going to put their lives on the line to try and save.
In short, your NPCs are where you need to focus your creative efforts, they are your levers to manipulate your characters. Build strong levers.
These tips are taken from my Scarier Horror Play book. You can get 40% off this book at DriveThruRPG using this discount link. You can get 40% off all new releases as a loyalty discount if you join my contact list.