Narcissa Black is a character full of contradictions. Coming from a family with a strong stance on pure - blood supremacy, she marries into another powerful pure - blood family, the Malfoys. Throughout the 'Harry Potter' series, we see her loyalty being tested. While she is initially associated with the Dark side through her family and husband, her love for her son makes her take actions that could be considered against the Dark Lord's wishes. Her character is a great example of how family and love can influence a person's actions even in a world filled with magic and danger.
Bellatrix could be a supporting character. She might have a complex reaction to Narcissa's pregnancy. On one hand, she could be happy for her sister, but on the other, her loyalty to Voldemort and the dark arts could make her view the new life as a potential weakness or a distraction. Another supporting character could be Narcissa's maid or house - elf. They would be there to take care of her needs during pregnancy, like preparing special meals and making sure the house is in order for the baby.
Draco Malfoy is often a very popular character in these fanfictions. Since Narcissa is his mother, his relationship with her and how it is affected by her interactions with Harry can be a major plot point.
Narcissa can be developed by showing her growth from a victim to a survivor. At first, she may be meek and submissive due to the abuse. But as the story progresses, she could become more assertive. For example, she starts to speak up against Lucius's actions in private, which is a small but significant step.
There could be other pure - blood wizarding families' members in the harem. For example, a member from the Greengrass family. And then there might be some half - blood or even muggle - born characters who add diversity to the harem, each with their own motives and backstories that interact with Narcissa's plans and the overall story.
Narcissa might be very angry with Harry's abusers. If it's the Dursleys, she could use magic to give them a taste of their own medicine, but in a more subtle way so as not to draw too much attention. For example, she might make their houseplants grow out of control or cause their food to spoil every time they're mean to Harry.
Bellatrix Lestrange could be a great addition. Her extreme loyalty to Voldemort and her relationship with Narcissa as sisters, and her hatred for Sirius as a blood traitor, would add a lot of tension to the story.
Voldemort could still be the main antagonist. Even if they travel back in time, his evil plans and his thirst for power would still be a major threat. He might sense something is amiss with Narcissa's behavior if she starts to act against him, and of course, he would see Harry as a huge threat regardless of the time period.
Bellatrix could be an interesting supporting character. She is Narcissa's sister, and her extreme views and loyalty to Voldemort could create some tension. For example, she might try to influence Narcissa's unborn child's future in a way that Narcissa doesn't like. Another good character could be Dobby. He could be secretly helping Narcissa with small tasks during her pregnancy, despite being a house - elf and the Malfoys' former servant.
Harry might have a more complex personality in this fanfiction. He may struggle between his loyalty to his old friends and his new life with Narcissa. His sense of right and wrong could be tested in different ways, for example, when dealing with Narcissa's former Death Eater connections. Narcissa would be a strong - willed character. She's used to getting what she wants in the pure - blood society. But with Harry, she has to learn to compromise and open up, which is a big change for her. And she would be constantly worried about how her actions will affect her family's reputation and safety.