As I said before, not knowing the specific 'jack in the box short story', it's hard to say. But most likely, Jack is a main character since he is in the title and associated with the box.
There could be a whole host of characters depending on the nature of the story. If it's a horror - themed 'jack in the box short story', there might be victims or bystanders in addition to Jack. For instance, if it's set in a haunted house and the box is part of the haunted artifacts, there could be a group of people exploring the house, and Jack is the terrifying entity that emerges from the box to torment them.
Well, if it's a typical 'jack in the box' story, Jack would be the primary character. However, there could also be other characters like the person who opens the box. Maybe it's a child who is curious about the box and then Jack appears. Or it could be an adult who stumbles upon the box and has an encounter with Jack, in which case both the finder and Jack are main characters.
It's likely that Jack is a main character. He could have friends or family members in the story as well. If the box has some sort of power or mystery, there might be other characters who are either affected by it or trying to figure it out. Maybe there's a wise old man who gives Jack clues about the box, or a young sidekick who joins Jack on his adventure related to the box.
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 primary character is Lazy Jack. He is likely to interact with other characters in the story. For instance, he could have a mother or father figure who tries to encourage him to be more industrious. There may also be neighbors or other people in the village (if it's set in a village) that he comes across during his lazy escapades.
The main character is surely Jack Frost. In some versions of the short story, there could be a human protagonist, perhaps a young girl or boy who ventures out into the snow and encounters Jack Frost. There might also be an old wise man or woman who knows the secrets about Jack Frost and tries to guide others in dealing with him.
The main characters are Jack and the giant. Jack is a young boy who is brave and a bit mischievous as he dares to steal from the giant. The giant is a huge, menacing figure who lives in the castle in the sky and tries to stop Jack from taking his treasures.
The main characters in the 'jack and jill short story' are Jack and Jill, of course. They are the two kids who go on the adventure of climbing the hill to get water.
The main characters are Jack, Jack's mother and the giant.
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.