It is a well-kept secret that the famous Harry Potter had a twin sister. I know, this sounds totally cheesy, but hear me out. Now, how it could possibly have been hidden that Harry had any immediate family remaining after that horrible night may seem impossible, but for at least the first three years, it wasn't too difficult. Annabelle Potter was a quiet child, generally fading into the background, especially when her aunt or uncle were upset (so almost all the time anyway). She slept on a mattress in a corner of the attic, and when she received her acceptance letter to Hogwarts, while Harry moved up an entire level, she was simply moved to the opposite corner. She didn't mind, though. No one went up to the attic besides Harry and herself, unless another of Dudley's toys needed to be put up there. And the rain on the roof at night was extremely delightful.
She went to Hogwarts with Harry, and for the first few years, she didn't have a very significant role in their adventures with Ron and Hermione. Her ideas were easily attributed to others in the group when the stories were recorded. This was done at Harry's request so that she could be protected from unwanted attention, especially after the stresses of the ensuing years and her shy nature.
Now, repeating the first three books would be tedious to write on my part and tedious to read on your part, because not much changes by adding Annabelle into the character list. But to give a general idea, she was usually included in Harry's adventures, although the incident of Harry and Ron flying to Hogwarts in the Weasley's car was completely their own experience, and in addition to the reprimands from the teachers, Harry received a small rebuke from his sister. Otherwise, the two had a very close relationship. If the reader would kindly reflect on how often Harry, Hermione, and Ron were together, Annabelle was with them almost every time. I would say that Annabelle and Harry spent more time together besides, but with how often the friends were together, there actually wasn't really any time that they could have done so.
Annabelle was a very bright student. She wasn't quite as smart or knowledgeable as Hermione, but she worked as hard for her grades and loved learning about as much. In fact, she and Hermione were best friends and would often study together, even asking each other random questions in the hallways, or during meals, or just about any time they thought they could catch the other off-guard. How they thought this game any fun was completely lost on Harry and Ron, and they elected to ignore it when it occurred.
And now we have reached the end of this summarization. The first few years of the Potters' adventures were about as uneventful as they could get with Harry being "The Chosen One." The real interesting stories start in about their fourth year, to be explained in the upcoming narrative.