The goddess could be a new character created specifically for the fanfiction. Maybe she's a being from an ancient magical civilization that has been dormant for ages.
She could be an ancient Forerunner who has been in stasis for millennia. The Forerunners were extremely advanced, so it would make sense for one of them to seem like a goddess with their technology and knowledge.
Well, it could be that the goddess helps Harry discover a new aspect of magic. She might teach him some ancient spells or show him hidden magical realms. Harry, in turn, could introduce the goddess to the wizarding world he knows, like Hogwarts and its unique traditions.
The goddess could also be an original creation for the fanfiction. A being with a unique set of powers like the ability to control time and space. This goddess could be betrothed to Harry to stop an impending doom that threatens both the wizarding and Muggle worlds. Her powers would be so great that she needs Harry's connection to the wizarding world to fully utilize them and save the day.
It could be Athena. Athena is known for her wisdom in Greek mythology. She could pass on her wisdom to Harry, making him not just a powerful wizard but also a very wise one. He could use this wisdom to solve problems in the wizarding world in a more strategic way.
The goddess could be an ancient Celtic deity. In the fanfiction, the author might choose a Celtic goddess because of the rich magical lore associated with them. For example, Morgan le Fay from Arthurian legend could be reimagined as a goddess in this context, and she could adopt Harry due to her interest in powerful magic users.
It could be an original character created by the fanfiction author. Maybe a powerful being from a long - forgotten magical realm.
She could be an ancient Celtic goddess, like Brigid. Brigid is associated with magic, healing, and smithcraft in Celtic mythology. In the fanfiction, she might see Harry's potential and decide to train him in her unique magical arts.
It could be an unknown character who has been in hiding all this time. Maybe she was once a powerful witch who made a pact with death and became its representative in the wizarding world.
It could be a completely original goddess created just for this fanfiction. Maybe she is the goddess of a forgotten element, like the goddess of lightening that has been long forgotten in the ninja world, but still holds great power.
It could be Athena. She is known for her wisdom, and Harry has shown great intelligence throughout the series. Maybe his strategic thinking and ability to solve complex problems come from his connection to Athena.