After losing his firefighter certification due to injury, he transfers to Ambulance 61 to keep working. Mills begins as a candidate on Truck 81, following in his deceased father's footsteps, but after proving his capabilities as a firefighter, he earns a spot on Squad 3. Charlie Barnett as Firefighter Candidate Peter Mills and later Truck 81, Squad 3, Ambulance 61 (seasons 1–3).She is memorialized by an inscription painted on the cab door of Ambulance 61. In "Three Bells", she was honored and remembered by her fellow co-workers at 51.
In the third-season premiere "Always", she was killed in its continuation of the second-season finale "Real Never Waits" cliffhanger, in which a serial arsonist intentionally set a trap for the first responders. Popular with her co-workers, she was openly gay, often cracking self-deprecating jokes about it. She and Kelly Severide shared an apartment as roommates and were best friends.
As an experienced paramedic, Shay was the designated ambulance driver and was well-liked by the mostly male crew of 51.
Casey is the Lieutenant at Firehouse 51 and the officer in charge of Truck Company 81. Jesse Spencer as Lieutenant Matthew Casey, Truck Company 81 (seasons 1–10 ).Main article: List of Chicago Fire characters Main