Kevin Hadley is a former firefighter that was assigned to Rescue Squad 3.
He joined Squad 3 prior to the start of the series and worked well with the team.
When he learned that Severide was grooming Peter Mills to join Squad 3, he wasn't too thrilled and went after him. It resulted in Boden forcibly transferring him. Hadley took the forced transfer personally, and began to target Peter Mills and Kelly Severide (Lieutenant of Squad 3). Hadley started fires at particularly difficult buildings that were listed on the "dangerous buildings" list. This lead to an investigation by the Chicago Police Department and the CFD Fire Investigations Unit.
Kevin Hadley was eventually arrested and found guilty of arson, he is now serving his sentence in prison.
He returned in "Three Bells", contacting Severide to come see him as he was still in prison. Whilst they were face to face, he claimed to have information on the arsonist who killed Shay that had been sent to him. Later an envelope with an address was sent to Severide, connecting the fire that killed Henry Mills to Shay's murder.