Maddy was the name of a girl that Number Nine meets in Chicago, and becomes the first person he ever really had extended interaction with. She is described as beautiful, around sixteen or seventeen, with straight black hairs and eyes to match and an athletic build.
At an unknown time, Maddy's father see's something that he should not have through his telescope and the Mogadorians kidnap him and his family. They have Maddy for a while before they decide to use her to find Nine, giving her ID's for recreational centres all over Chicago.
She first catches Nine's attention as he is about to try the climbing wall at the Windy City Wall recreational centre, his sweaty palms cause him to slip and fall and he is embarrassed.
Maddy later approaches Nine and gives him her phone number. They arrange a date at the Planetarium; Maddy tells Nine about her parents, her father an Astronomer and her mother a philosophy student who recently picked up a job at the University of Chicago, Nine notes that she sounds sad when she talks about them, as if they aren't around very often.
Maddy bumps into Nine, not by accident, in a thrift store and takes a picture of him, presumably for the Mogadorians.
Nine picks Maddy up in one of Sandor's cars, for their second date. Mogadorians suddenly appear and try and ambush them but Nine manages to get away from them in a high speed chase through the city. Maddy is angry and upset and storms off once he stops the car.
Maddy texts Nine again and tells him that they can try again, without cars. He goes to her house to apologise in person. They talk for a while and fall asleep. When Nine wakes up he is surrounded by Mogadorians and in the struggle finds Maddy's collection of rec centre ID's. Nine gets to Maddy and discovers his Legacy Transference allowing her to run along the walls with him, they almost escape but Maddy tasers him and he is captured.
In the van on the way to West Virginia, the Mogadorian in charge of the Chicago Scout Team explains Maddy's betrayal and that she was forced to work for them so that her parents would live.
When they arrive at the Mountain Base, Maddy and her Parent's are reunited but a Piken is released into the room with them and Nine watches as she pleads for his help before they are all killed.