Ella is Libby and Frank’s daughter. Keira is her aunt and the one who named her. Her full name is actually Carrie Ella. We learn that in the Hellbeasts’ Mistake.
Growing up, her aunty Kaz (Kiera) used to call her Ella Bella after her obsession with Beauty and the Beast. Then when Fae was born, that changed to Ella Belly as she used to blow raspberries on her belly when Fae was little. That name sort of stuck, and her family, including her cousin Faith, called her Ella Belly even into adulthood.
Ella’s story really picks up in the Hellbeast King series, where she meets Jared Cerberus. She first met him by chance when she was just 16 years old. She doesn’t meet up with him again for many years later, long after she’s out of college and enjoying her life working with animals as sort of a park ranger of sorts.