In some cases, it might be well - known characters from the original work (if it's a fanfic based on an existing series). For example, if it's a fanfic related to a fantasy book series, it could be the main hero or heroine of that series who has to endure a 'birth by fire' situation to fulfill their destiny.
In the 'birth of a hero novel', the main character is obviously the hero. This person could be a young boy or girl with extraordinary potential. There may be their family members as well, who play important roles in their early life. For example, a father who passes on some valuable skills or a mother who instills moral values. Additionally, there could be friends or comrades who join the hero on their adventures, and an enemy, perhaps a powerful villain with their own motives and goals that conflict with the hero's.
I'm not sure specifically as I haven't read it. But it might include characters related to the historical events or stories that the original 'Birth of a Nation' was about.
Obviously, Ana herself would be the central character. Her partner could be an important character too, being there for her during the pregnancy and birth. Maybe her mother or a close female friend who offers advice and support.
There could be visionary leaders. These are the characters with a grand plan for the new world. Then there are the ordinary people who get caught up in the events and end up contributing to the new world's birth in their own small ways. Also, there might be some sort of magical or supernatural beings that play a role, like spirits that protect the new world as it is being born.
There could be other characters too, like the Sage of Six Paths. He might play a crucial role in the Juubi's birth in the fanfic. And also, villains from the Naruto universe could be involved, as their actions might have either directly or indirectly led to the birth of the Juubi.
In the Birth of King Arthur story, Merlin is a crucial character. He is the one who uses his magic to bring about Arthur's birth. Arthur's mother also plays an important part. She is often in a situation where she has to keep Arthur's identity a secret at first. And Arthur, as the future king, is the main focus. There are also some minor characters like Sir Ector who plays a role in Arthur's early life when his true identity is not known yet.
Most likely Naruto would be one of the main characters, given that it's in the title. There might also be other characters from the Naruto universe like Sakura or Kakashi who could be involved in Naruto's adventure related to the forest's birth. Maybe even some new characters created specifically for this fanfiction, like forest spirits or guardians of nature.
In the birth of Jesus story, there are quite a few important characters. Jesus is the main one, being the Son of God. Mary, a young and devout woman, was chosen to be his mother. Joseph was an important figure too. He was a righteous man and was there to support Mary. The shepherds played a role as well. They were simple folk but were the first to be told about Jesus' birth by the angels. And the Magi, these wise men from the East, they were important as they brought valuable gifts and recognized the significance of Jesus' birth from afar.