Jody Mills is the sheriff of Sioux Falls, South Dakota. She is an ally of the Winchesters, and occasionally helps them in their cases. She was also affiliated with Bobby Singer before his death.
Background
Jody was married and had a son, Owen, who died of unknown causes some time before the Apocalypse. She had a history with Bobby Singer, who she typically regarded as a troublemaker and a drunk.
Season 5
She first appears in Dead Men Don't Wear Plaid when she walks into the diner and sees two men talking to "Digger". She asks Sam and Dean who they are, and they reply FBI. When they give her their "supervisor's" number, she finds out it's in fact, Bobby Singer, by recognizing his voice. She warns Sam and Dean to stop whatever it is they're planning to do.
She later arrests Sam and Dean when they go after a man who had come back from the dead. Bobby later comes by and talks her into releasing Sam and Dean. It is later revealed that her own son had come back, which is the reason she is not taking action on this matter.
Then her son starts to get sick and she is very worried. But when she walks back into the room, she finds her husband dead, and her son eating him. As she backs away, Sam comes in and saves her. He gets her out, and she tries to go back and save her husband, but Sam tells her that her husband is already dead. Sam goes in and shoots her son. So she teams up with Sam and gets the town ready for the Zombie attack. She and Sam later arrive to save Bobby and Dean. After the attack, she and Dean sweep the area for any remaining zombies.
Season 6
Jody Mills returns
She later returns in Weekend At Bobby's, when she and an FBI agent arrive at Bobby's door. She helps Bobby by getting the agent outside his house, but is shocked to hear that he also had a body outside as well as inside. They go outside to find a hole, and she gets rid of the agents. Bobby later asks her a favour to get Rufus moved to jail in town. Jody is shocked by this request and tells Bobby she could lose her job and drives away. She later comes to Bobby's doorstep with Rufus, and tells Bobby he has a certain amount of time before she calls in.
Season 7
Jody follows Dr. Gaines
In Hello, Cruel World Jody was at the hospital the Leviathan decided to use as their feeding area. She recently had surgery and was sharing a room with another woman. In the night, Jody witnesses the woman being taken away by Dr. Gaines who treated her. She follows him to see Dr. Gaines kill and eat the woman. Escaping back to her room, Jody soon passes out in the hallway, and wakes up back in her room with Dr. Gaines telling her he'll see her soon. With her life in danger, she calls Bobby Singer for help. Bobby gets her out of the hospital and checks it out.
In Slash Fiction, Jody manages to find Bobby in his hideout. She offers him food and beer. Jody thanks Bobby for saving her and offers to cook for him. Its thanks to Jody that Bobby figures out what can hurt the Leviathans. Jody was cleaning the place using a chemical that can hurt the Leviathans, and Bobby kisses her for it. As she leaves, Bobby gives her the Leviathan's head and tells her to toss it into the river.
In Time After Time she calls Sam up with a case of people being rapidly aged. When she calls a second time she finds out that Dean has gone missing, and offers her help. Sam asks her to go to Bobby's storage to grab some boxes. She arrives at the house Sam is staying at and brings in the boxes. Jody and Sam look over the case and from the ring on the man's finger they figure out it's the Time God Chronos. So they look for a way to summon Chronos. Eventually, they find the ritual required, and Sam performs it, summoning both Chronos and Dean to the Present Day. Sam stabs Chronos with an olive stake killing the God and saving Dean.
Season 8
In Sacrifice, Jody goes on a date with what she believes to be a man named Rodrick, but it is actually Crowley. The two get along great and Crowley claims to have lost loved ones, causing Jody to remember her dead son and husband and cry. She goes into the bathroom to calm down, but Crowley casts a spell that causes to her to start choking to death on her own blood. He then calls up Sam and Dean, and tells them he will only spare Jody's life if they stop the trials to close the Gates of Hell and turn over the demon tablet, using the short time Jody has left to press the issue. Dean agrees if he swaps the angel tablet instead and Crowley tells them they must say "I surrender" before he stops. Presumably Sam and Dean do and Jody is saved.