It's well documented that Crookshanks belonged to Hermione Granger from very late August of 1993. He was her companion all through school, she his Mistress and he, her loyal familiar. However, four short years later, Crookshanks' mistress was forced to leave him in the care of her family friends. It also well known that Hermione and Crookshanks were reunited following the following summer.
What you weren't told is that Ginny Weasley actually took him to Hogwarts for her seventh year as a makeshift pet. Crookshanks was relatively happy living in the castle he was so used to and following mostly familiar faces around.
Although, he did miss his mistress and her two friends. He liked the dark-haired one. The other one, though... if he could ever get up to his dorm, Crookshanks knew he'd leave a little "homemade surprise" on that one's pillow. He hadn't forgotten the number of kicks aimed his way that one year.
Regardless, when things started to get loud, Crookshanks knew, like the good boy he was, to hide under his temporary caretaker's bed. He was safe there and remained there until all the loud noises stopped. And they did for a bit.
Crookshanks dared to venture forth, and when the noises started again, he hid with Mrs. Norris, the pair having a mutual understanding of one another. The noise stopped again, and the quiet murmuring of the castle returned. Albeit there was a new energy to be found.
When Crookshanks started wandering the castle again, he found it very different than before. It was dusty. There were holes where there hadn't been before. It smelled like death as well.
Then Crookshanks found a room he hadn't been in before. There were bodies laid there. They smelled like death and decay, and one pale faced one looked deformed and odd. Crookshanks knew this was the person who had haunted his mistress and her friends. He smelled vaguely like his mistress' dark- haired friend. He sat on the corpse's chest and sniffed it.
He turned on the spot and decided he was going to leave that "homemade surprise" for this one. Crookshanks lifted his tail and deposited a nice brown pile on the deformed and pale face of the body before scampering into the Great Hall. He saw his mistress and bounded to her.
"Crookshanks! There you are. What a good boy." She cooed to him as she picked him up and stroked his fur, and he purred contentedly on her warm lap.