The main theme might be about power and its loss. Since the character was once a king, perhaps the story shows how he went from a position of great power to something else, like an ordinary person. It could also be about the lessons learned during that transition.
Well, it could be centered around the idea of identity. Being a king gives one a very distinct identity, and when that's taken away, as the title implies 'once was a king', the story might explore how the character redefines himself without that royal identity. It may also touch on the relationships the king had when he was in power and how they change.
One possible theme could be power. Since it's about a king, power dynamics are likely involved. It could show how power can corrupt or how it should be used responsibly.
The main character is most likely the former king himself. Since the title focuses on the fact that 'once was a king', it's reasonable to assume the story will revolve around him and his experiences.
One possible main theme could be achievement. If there is a character known as the 'king of kites', it may be about his or her achievement in the field of kite - making or flying.
A common theme could be power. Kings have a lot of power and stories might revolve around how they use it, whether for good like protecting their kingdom and people, or for bad like being a tyrant and oppressing the subjects.
It could also be about the power of kindness. Suppose the king was cold - hearted at the start, but through interactions with kind - hearted people in the story, he became a more benevolent ruler. Then the moral would be that kindness has the power to transform not only individuals but also the whole kingdom or the situation around them.
The king in this short story is the focal point. He may have certain goals or desires. Maybe he wants to expand his kingdom, or perhaps he is more concerned with maintaining peace within his existing territory. His character traits will determine how he pursues these aims. For instance, if he is brave, he might lead his armies into battle personally. If he is wise, he will seek counsel from his advisors before making important decisions.
The main theme could be power and manipulation. The king might represent power, and Jezebel could be a symbol of manipulation, using her influence over the king for her own ends.
The main theme could be about the power of nature. The frost king might represent the might and beauty of winter and its icy realm.
Another theme might be the power of imagination. Maybe the little girl has a vivid imagination that helps her deal with the world around her or even creates some sort of adventure in the story. The story could also have a theme of family. If there are interactions with her family members, it could be about the importance of family bonds, how they support her or how she learns from them.
If the king was a good one who helped his people, the moral could be about the importance of leadership and kindness. That leaders should care for their subjects.