Ella is Dr. Martinez's daughter. She is Max's younger half-sister, she met Max in The Angel Experiment.
In The Angel Experiment, Max saw Ella fighting with three other boys and defended her. Ella ran from the scene, but the boys chased after Max with guns. A bullet hit her shoulder. When Max escaped, she found Ella's home and ended up resting there while her wing and shoulder were healing.
Later in that book, while in New York City, the flock visits a library. While signing in, Max uses Ella's name.
In Max, there are few mentionings of Ella. It says that she called the flock about her missing mother when she discovered that Dr. Martinez was kidnapped. Ella was also present when Max, Angel, and the Krelp rescued Dr. Martinez.
In Angel, it states that Ella was reading to Iggy. It is also apparent in the book that Ella has a crush on Iggy.