Summary: Ginny feels at home but she's still absolutely alone.
Story:
Ginny feels so at home at Hogwarts. She loves her bed curtains, she loves the classroom desks, she loves the castle walls. Hogwarts was the safest place on earth. She'd heard Harry say it when telling a story from his first year and everything Harry said must be real. So Ginny accepted Hogworts' protection and comfort. It helped her deal. It helped her grasp with the perpetual loneliness.
Ginny lies on her gigantic bed. It engulfs her. She likes it.
Ginny opens her worn diary. It engulfs her. She likes it.
Ginny came to school and she didn't even have to try to feel at home. Hogwarts was home minus her mum and adding on the endless amount of castle rooms. So less rules and more places to explore. It worked. Especially because she was a Weasely. She belonged in Hogwarts because of every family member before her who had explored Hogwarts and known that every step they took down the hidden pathway or into the Gryffindor tower was one more adventure that the Weasely blood enabled them to handle.
Unfortunately you can feel at home somewhere and still feel absolutely alone because fitting natuarally doesn't mean fitting in. Ginny was alone. Those Weaselys who had paved the way for her at Hogwarts had left her to travel on her own. The adventures that Ron was having left him too starstruck to gaze upon his younger sister. And when Ginny tried to approach Fred (he was always the nicer twin) he was already on his way to meet with Lee and George for an important joke planning session. Ginny wasn't invited. (they didn't need her to keep mum away from their rooms anymore with all the empty spaces Hogwarts provided.) Percy was too busy. Percy had never been too busy for Ginny before Hogwarts but was now.
Ginny is left alone and bewildered. She always counted on her family being around her and with her. At home she was always surrounded a family who held the youngest and single sister as a miracle. They all loved her and amused her to all ends. Every relationship she established with her brothers was personalized to fit her needs. And they let her because she was their baby sister. And then when they left mum told her that her brothers were just making sure that Hogwarts was good enough for their little Ginny. That sustained her up until her final brother left. When Ron was gone Ginny almost sunk into loneliness, but mum told Ginny that she had much to prepare for school. Mum needed her at every moment just because Ginny needed her. That's how mum was. She taught Ginny as many basic spells as she would need and let her pick out one special hex to learn. That was right after Bill had written from Egypt about the famous Bat Mummy that he had sneezed on and gotten his boogies turned into bats. He also said he was coming home early. Ginny adored Bill and knew he would appreciate her learning the one spell that had gotten the better of him. When she performed it on him on his return he laughed till he cried. Then till the summer Ginny had Bill to amuse her. He regaled her with so many stories of his Hogwarts days that by the time September 1st came around Ginny was prepared even without the day-by-day account Ron had given her when he returned of his amazing time at Hogwarts and his relationship with the amazing Harry Potter (and Hermione Granger which he talked about almost as much as he did of Harry).
So when Ginny arrives at Hogwarts she imagines she knows it all. But she doesn't. Because she doesn't know that her brothers will no longer hold her over everything else in their hearts and minds. Arriving at Hogwarts Ginny has to make a personal readjustment. Ginny didn't count on any readjustments. And she refuses to make any changes that she never thought she'd need. So Ginny gains the status of a loner, unapproachable because she never needed to give an invitation. He brothers had always been in her life and they were all she needed.
In one moment of weakness she writes to Bill. She never tells her mum. Ginny knows that Hogwarts doesn't need her mum, even if Ginny needs her. And Ginny is at Hogwarts now.
Bills response has four letters, one for Ginny and the other three each for her brothers. Ginny throws them all out. By that time she has her diary and Ginny doesn't need anyone else. Tom and she can explore Hogwarts together, own Hogwarts together. It's now her domain and Tom is her ruling partner.
She likes it.