Not having read the story, it's pure speculation. There might be an old wise figure who knows something about the nightmare box's origin. And then there could be a young and reckless character who ignores the warnings and opens the box, setting off the chain of events in the story.
I'm not sure as I haven't read the story, but it could be a passenger who is experiencing the nightmare.
As I don't know the particular 'the box short story', it's hard to say. It could be a young protagonist who discovers the box. Maybe an old man who has been guarding the box for years.
The two most important characters are Jack Skellington and Santa Claus. Jack is the main character who drives the story forward. He's the one who discovers Christmas Town and tries to take over Christmas. Santa Claus is also key as he is the rightful figure associated with Christmas. Jack's actions affect Santa, like when he kidnaps him. There are also other characters like Jack's minions in Halloween Town who play a role in trying to execute Jack's Christmas plan.
The main characters are Jack Skellington, the Pumpkin King of Halloweentown; Sally, a rag - doll created by Dr. Finkelstein; and Santa Claus. There are also other characters like the citizens of Halloweentown and Christmas Town.
Well, without having read 'the nightmare box short story', it could be that the story focuses on the discovery of this so - called nightmare box. It might explore the reactions of the characters who find it. They could be curious at first, and then as they start to understand its connection to nightmares, they become frightened. Perhaps there are attempts to destroy the box or seal it away to stop the nightmares from spreading.
In the'monster box short story', the main characters could be diverse. It might be a curious explorer who comes across the box during a journey. This character would be the one to initiate the events related to the box. There could also be a local villager who warns the explorer about the box but is ignored. And then, there might be the 'creator' of the box in the backstory, who could be either a mad scientist or an ancient sorcerer, and their motives for creating the box and putting monsters in it could be a big part of the mystery that the main characters have to unravel.
Since I haven't read the story, it's hard to say for sure. But it might be an old man who has lost hope in life until he stumbles upon this box. The old man could be one of the main characters. Or it could be a group of people, like a family, who find the box together and all have different reactions to what's inside, making them all important characters in the story.
The main characters in the 'black box short story' really depend on the nature of the story. If it's a story about a lost civilization and the black box is a relic from that time, the main characters could be archaeologists or historians. They would be the ones trying to unlock the secrets of the black box. There might also be local guides or informants who play important roles in the story. If it's a more fantastical story, it could be a wizard or a magical being who is either protecting or seeking the black box, and there would be other characters in their world who interact with them regarding the black box.
There could be a group of characters. Maybe a scientist or an investigator who is intrigued by the black box. And there might be others around them, like friends or colleagues, who either help or hinder their exploration of the box. For example, a friend might encourage the main character to open the box while a colleague might warn against it due to potential dangers.
Since I don't have the full text of 'the black box short story', it's hard to say precisely. However, it's common to have a central character who is affected by the presence of the black box. There could be a secondary character who is somehow related to the origin of the box, like a previous owner or a creator. And there might be a character who serves as a foil to the main character, highlighting different attitudes towards the black box.