Kai Barton is the oldest daughter of Clint and Laura Barton. Kai was Clint's biological daughter, but when he and Laura married, Laura adopted her and became the mother the girl had always needed. Kai was much like her father, meaning she was an amazing archer, but from a young age, she learned how to fight because of her aunt Natasha.
When Kai turned fifteen, her father had been taken under control by Loki, so Natasha promised the young girl and her family that she'd bring Clint back. Nick Fury had other plans though, since he knew how amazing Kai was with a bow and arrow, as well as in a physical fight, he recruited Kai to help defeat Loki. She worked beside her father, once Natasha had gotten him past his mind control.
Three years later, Kai was an official Avenger, much to Laura's dismay. Laura loved Kai, and she was scared Kai would end up hurt or worse because of the Avengers, much like she had worried for Clint. What the parents didn't expect was Kai to find someone she would find happiness with as much as she had her own family...
You're Tony Starks daughter that he didn't know about. You get into a fight and are sent to live with him, you know very little about him, and he knows very little about you. You are seventeen and graduated high school. You are a genius with an IQ of 187 and can read 20,000 words per minute with an eidetic (photographic) memory. So, you're really smart. You don't get along with Loki, Peter Parker is your best friend, and you love to spar with Bucky and Steve from time to time. The Avengers soon become like your family, and you slowly start to warm up to Tony, as for Loki, the two of you are constantly at each other's throats.