Well, it could be a curious child. The child is always exploring and ends up getting involved in the horror of the '7 second riddles'. Maybe they are the ones who first encounter the strange and terrifying things.
Another possible main character could be a psychic. In the '7 second riddles horror story', the psychic can sense the evil before others. They are constantly warning people but are often not believed until it's too late. They have to use their powers to try and fight against the horror that unfolds every 7 seconds.
There might be a skeptic adult. This adult doesn't believe in the horror at first but as they get caught up in the '7 second riddles horror story', they become one of the main characters. They are constantly trying to rationalize what's happening but fails miserably.