Something something TTRPG newbies
Bystanders are the everyday folk the hunters will encounter in the world of the game. You'll likely create a couple when writing a mystery, and you'll probably have to invent new ones over the course of the story. If you intend to run a game, I suggest starting a list of interesting names so that you have them handy for this.
Hunters are, as the name suggests, the people doing the monster-hunting. I use the term interchangably for the real-life people in a gaming group and the characters they play. You could also call them players or PCs, but where's the fun in that?
The Keeper is the person who creates mysteries and presents them to the hunters. This is MOTW's cool name for a GM or DM.
Minions support monsters. They can be less powerful versions of a monster, ordinary animals under a monster's mind control, a sympathetic human trying to protect a monster, or your typical evil henchmen.
Monsters are the main antagonists of mysteries. They might be creatures of folklore, aliens, eldritch abominations, or humans with magic abilites. Whatever the case, they're the ones driving the plot forward. Monsters don't have to be pure evil, but their actions should have dangerous consequences if left unchecked.
A mystery takes up one session of play. In a typical mystery, the hunters will discover the presence of a monster, invesigate to find details, and finally, confront the monster. If a Monster of the Week game is like a TV show, then a mystery is one episode of that show.
Phenomena are an alternate option for the main threat of a mystery. WIP
This is the catch-all term for the elements that make up a mystery. This includes monsters, minions, bystanders, and locations.