In practice, the only requirement for being a Slytherin is be mean. I, Rachel Greengrass of Slytherin, think this is unfair. As you'll see in my story, I've only ever beaten a few people to a pulp, and honestly, they deserved it.
Sirius Black was once a father, and he enjoyed this role. But when he was blamed for the death of his best friend's family and sentenced to Azkaban, where was his daughter to go? More importantly, how could she understand everything at five years old?