Please read this. It's not part of the story, but it will help you to understand the background a lot better.
JK Rowling owns all characters apart from Amelia and a few others.
I have created my own version of the Harry Potter world to fit this story. An alternate universe. There is no Lily, James or Harry Potter. (Sorry!) Amelia Baxter is 19 years old and in her last year of Hogwarts, along with Ron and Hermione. Her parents were killed by Voldemort when she was one year old. Voldemort and the majority of his Death Eaters were then defeated by Albus Dumbledore and the Ministry of Magic's Aurors. Amelia was then, having no family, put into a care home until she left at the age of 12, in her second year at Hogwarts, after an invitation for her to go and live with Hermione and her parents. Having been brought up in a Muggle care home. There was not much difference. She had, however, been disliked at the care home and had not enjoyed her time there. Although she feels no resentment to the home, she was glad to leave.
At Hogwarts, her best friends are (Obviously) Hermione and Ron. But she also has a close relationship with Professor Minerva McGonagall and looks up to her almost as a grandmother, and often seeks her advice. She is an extremely gifted witch who excels in Transfiguration, Defense against the Dark Arts, Charms, Care of Magical Creatures and Herbology. She is also unnaturally gifted at Potions and has a clear understanding of how different ingredients work together and what effect they have. However, she occasionally messes up in lessons to avoid much extra attention.
As I have said, there is no Lilly Potter in this story. (Sorry to all Snape/Lilly fans!) So Severus Snape does not pine after a lost love. He is 22 years old and has stopped all contact with his widowed Mother, who blamed him on his Father's death and openly expressed her resentment towards him. (Just to be clear, his Father, in my story anyway, died in a fight he started in London after getting drunk. So NOT Snape's Fault. His Mother is just a bitch.) He is close friends with Albus Dumbledore and looks to him if he is ever in need. He makes good money from his job at Hogwarts and inherited an impressive as-yet-untouched sum from his Father's death.
So... Yeah, that's pretty much the basics. There are probably a few things I have changed that are self-explanatory. But I think I have covered what you need to let the story make sense.