Zee Hatcher was Alex Sawyer's friend throughout the series, managing to survive alongside him past the fifth book, Execution.
Zee Hatcher is described as shorter than Alex, but wider, meaning slightly broader chested than Alex. It is mentioned that he has dark hair, but not specified whether that meant brown or black hair. It's mentioned on multiple occasions that Zee has an accent, but not specified what sort, so the type of accent may influence his appearance slightly.
Zee is almost always upbeat and happy. He's humorous and quick witted, and makes a great comic relief. Even in very stressful times, like in the pursuit of the Blacksuits, he's always got a joke ready. Alex describes on many occasions that he couldn't have escaped without Zee, not only because of his physical contributions, but because his optimistic attitude towards life overall kept everyone's spirits high. When even Zee gets quiet, either with sadness or nervousness, you know you're pretty much screwed.
Aside from his almost constant optimism, Zee is very hotheaded. If when he's annoyed, he can snap very easily. When he comes into contact with Simon, this becomes a problem. Though the two are forced to work together, they don't always get along the greatest. As a side effect of his hotheadedness, he's very protective of those who he cares about. He volunteers to stay behind in the control room so that his friends to get out, while Alex and Simon fight about who stays behind.
Zee is best friends with Alex and Donovan. The three of them form a main trio that can be seen hanging out through much of the first book. They do almost everything together (SPOILER before Donovan gets taken SPOILER) and planned to be the only ones to escape Furnace Penitentiary.
In Solitary, Simon saves Zee and Alex from Solitary Confinement. Though they're friends, they don't get along quite as well as Zee and Alex do. On multiple occasions, Zee has small arguements with Simon, nothing too huge, but enough to say that the two of them wouldn't get along well.
In Fugitives, Zee, Simon, and Alex meet a girl named Lucy Wells in the subway. She begins to hang around with the gang (after some coaxing and reassuring), and begins to form romantic feelings for Zee. By the end of Execution, Zee and Lucy are boyfriend and girlfriend.