In some fanfictions, the mother could be an original character created by the author. This allows for a lot of creative freedom in developing the backstory and relationship dynamics.
There's also the possibility that it could be an original character created just for the fanfiction. This would give the writer more creative freedom to shape the story around the relationship between Batman and the mother, and how it impacts their son.
Annabeth could be the mother. Since she and Percy have a strong relationship, it would be a natural choice. Their son would inherit both their intelligence and combat skills.
In many fanfictions, Sakura could be a potential mother. Since she has a close relationship with Naruto in the original series, some fans might imagine a scenario where they end up together and have a son. Another possibility could be Hinata. Given her deep love for Naruto, it's not hard to fathom a fanfiction where they have a secret son.
Another option is that the mother is a mysterious witch from a far - off land. Sirius might have met her during one of his travels when he was on the run from the Ministry. They had a short - lived relationship, and she left the baby with Sirius because she knew he was from a powerful wizarding family and could protect the child better in the wizarding world he knew.
The Death Eaters could be antagonists. Even though Regulus turned against them, they might not forgive his betrayal and try to harm him or the school where he teaches. They could send spies or launch attacks to get revenge.
It could be a power - hungry Slytherin who doesn't like that Regulus is showing an interest in Gryffindor values. This Slytherin might see it as a betrayal to their house and try to stop Regulus by any means necessary, even if it means using dark magic.
It could be a northern girl, someone who Brandon met during his travels in the North. Maybe she was a daughter of a minor lord, and they fell in love while he was on a mission to secure the North's borders.
It could be Hinata. Even though in the original story they are openly married, in a fanfic scenario, there could be a situation where their relationship was kept secret for some time and she gave birth to a son secretly. Maybe due to some threat to the village or Naruto's safety.
Neville Longbottom might also be a character. Neville has his own connection to the fight against the dark side. He could share his experiences with her and they could bond over their shared desire to break free from the dark legacies of their families. Also, Luna Lovegood could be in the story. Luna's unique perspective on life and magic would be a great contrast to the daughter's more complex family situation, and they could have some very interesting conversations.
Luna Lovegood's child could be a great pairing. They could have an interesting dynamic, with Luna's child having a very different and whimsical view of magic, which would contrast and complement the son of Harry and Regulus. Another good character to pair with him could be Neville Longbottom's son or daughter. Given Neville's history of standing up to Voldemort, his offspring could bring a sense of courage and plant knowledge that could be useful in adventures with the son of Harry and Regulus. And of course, Ginny Weasley and Harry Potter's own children could be paired, as they would have a natural connection through their family ties.