It could be a completely new character. Maybe Naruto met her during his travels, and for some reason, she had to entrust the baby to Naruto. This could be due to her being in danger or having some kind of mission that made it impossible for her to take care of the baby herself.
Another possibility is that it's someone from Naruto's past that we haven't expected. It could be a character who had a brief encounter with Naruto long ago, and the relationship developed secretly. The circumstances led to the birth of the baby, and now Naruto is back with it.
It could be Hinata. Since they are in a relationship in the original series, it would be a natural choice. Hinata would be a loving and caring mother, and her gentle nature would be a great influence on the baby.
It could be Hinata. Since they got married in the original series, it would be a natural choice for her to be the mother in the fanfiction.
One possibility is that Naruto has been on a long and secret mission. During this mission, he somehow ends up taking care of a baby. Maybe the baby is an orphan from a village he saved. Naruto, with his big heart, couldn't leave the baby behind. So he returns to the Hidden Leaf Village with the baby, causing a stir among his friends and the villagers. Some might be worried about how he can handle being a ninja and taking care of a baby at the same time, while others are just happy to see him back.
It could be an unknown character. Maybe Steve had a one - night stand with someone while on a trip out of town and she left the baby with him without any explanation.
In this fanfiction, the mother could be a powerful sorceress. She used her magic to create a baby with Kratos' essence for some grand purpose. But then she disappeared, leaving Kratos to deal with the baby. This would add an interesting layer of mystery to the story as Kratos tries to figure out why she did it and what the baby's true nature and purpose are.
Perhaps it could be a character from an alternate dimension. In some fanfiction scenarios, portals open between different dimensions, and Spiderman could have had an encounter with a female character from another world, which led to the baby.
It could be someone from Stiles' past that he had a brief relationship with. Maybe a girl he met during a summer vacation or a girl from another town that he had a fling with and then lost contact with until she shows up with the baby.
Another option could be an original character created just for the fanfiction. This gives the writer a lot of freedom to create a character with unique traits and a backstory that fits the plot they have in mind for Draco and the baby.
The baby could be the child of a fallen ninja. Maybe the ninja was an ally of Naruto's and before dying, entrusted Naruto with the baby. This would add an emotional depth to the story as Naruto would feel a great sense of responsibility to raise the baby well.
One possible character could be Jiraiya. He was Naruto's mentor later in life, but in a fanfiction, he could have taken Naruto as a baby. He had a close relationship with Naruto's father, so it would make sense for him to take Naruto under his wing from the start. Another could be Tsunade. As one of the Sannin, she has the power and the means to protect Naruto. Plus, she might have seen it as her duty to take care of the son of her old teammate.