After Regulus's sudden disappearance or else as it was rumored; his death by the hands of Voldemort for betraying him, Cassandra Dawson (Regulus's wife) died of shock and despair, leaving behind only a year old daughter named Katrina. But her uncle: Sirius who loved his niece more than anything in the world, took care of her until the age of two when unfortunately he was also sent to Azkaban for the crime he did not commit.
However, Cassandra's three-years older sister Demetria who was married to a muggle and had a four-year old son named Thomas, gladly took her in. Katrina's, or Kate as she liked to be called, aunt and uncle provided her with the love they would have given to a daughter if they had one, and Tom gave her the love and protection of an older brother.
As Kate grew up, she started to wonder where her mother and father were, Demetria didn't hesitate to tell her the truth about their untimely deaths because she knew that Kate deserved to know the truth without any doubt. She was told about her father; about how he was killed by an evil, dark, wizard named Voldemort because of his wrongdoing and foolishness of joining the dark side, and she had to believe it because there was no proof not to. She grew up always wishing and trying to never be like her father, to never do something he did. As the years passed by, Kate discovered a rather odd ability about herself; she could talk to snakes.
When she told her aunt and uncle about it, instead of seeing them amazed, they had a look of terror on their faces and they told Kate that she was a Parseltongue but talking to snakes wasn't a good sign even in the Wizarding world. Very very few people had the rare gift of speaking to snakes, so she was told to keep it a secret and never openly talk about it in front of anyone, and of course she did as she was told and never spoke of it.
When Thomas, her cousin, turned eleven, his mother decided to send him to Durmstrang Institute instead of Hogwarts. Kate waited for the remaining two years for when she would be getting her own letter from Hogwarts, and she was very eager about it since she had heard all about Hogwarts as she continued with muggle school for the following years.