Cassiopeia needed a place to stay. She and Draco had been living on the street until he got arrested for panhandling, and she split. Orion was in Kansas or something. She tried finding Victory or Kannika, but didn't know where they were, so finally, she tracked Aunt Priss' scent to a man named Rupert Giles. She tracked him to Cleveland, and began investigating the Slayer School. She didn't want to come forward about who she was. Nobody trusted Drusilla's offspring, and without Sabre to vouch for them, they were damned. Not even two years old, and homeless, orphans. So she checked out the school. She could adjust her human form, whenever she wanted. She looked about 17, but not always the same girl, able to adjust her skin, eye and hair colour at will.
So she pretended to be a student. It was just a spur of the moment thing at first, but then she started to like it. SHe talked with the girls, blended in, went to classes and lessons and took weapons training, then, at night, she shifted into cat form, and slept in the stables.
Willow knew she was there. She observed her from a distance, and decided to let her go along with it for a while at least. She decided to give her a little time before letting Giles know. If the girl was willingly sleeping in a stable, things mush be pretty hard, and she didn't want to spook her into doing something rash. Cassie curled up in the hay and went to sleep.