The main characters would be the Scarlet Witch as the mother - like figure. The 'little Harry Potter' would be the central focus in terms of the story's development related to the Harry Potter elements. Then, we might see some of the Hogwarts professors. For instance, Dumbledore could play an interesting role. He might be initially cautious about the Scarlet Witch's presence in the wizarding world but then come to see the value of her protecting the young wizard. And of course, Voldemort could be a lurking antagonist, creating threats to both the Scarlet Witch and the boy she's protecting.
Obviously, the Scarlet Witch would be one of the main characters. And then there would be a character inspired by Harry Potter, probably a young boy with a lightning - shaped scar on his forehead. There could also be other characters from the Harry Potter universe like Hermione and Ron, who might interact with the Scarlet Witch and the 'little Harry Potter' in different ways.
The Scarlet Witch is the key character here as she takes on the mother role. The 'little Harry Potter' character is essential too. But we could also have some new characters created specifically for this fanfic. Maybe a magical creature that is drawn to the Scarlet Witch's unique magic and becomes a sort of helper to the duo. And then there could be a group of dark wizards who are jealous of the Scarlet Witch's power and try to use the 'little Harry Potter' to get to her, adding more conflict to the story.