The Winchesters and Jody Mills

Jody Mills is an ally and friend (and occasional maternal figure) to Sam and Dean.

Background
Sam and Dean encountered Sheriff Jody Mills for the first time during a visit to Bobby Singer's hometown, Sioux Falls. The brothers had intended to use Bobby as their superior so Jody wouldn't interfere with their investigation while they posed as FBI agents. Jody recognized Bobby's voice and immediately told the brothers off.

When townspeople started becoming zombies, Sam and Dean helped Jody deal with the problem and since then, Jody has requested the brothers help on cases with a supernatural vibe to it. Jody has also become a hunter and one of the few allies of Sam and Dean who is still alive.

Season 5
In Dead Men Don't Wear Plaid, Sam and Dean travel to Sioux Falls to investigate a murder. They soon discover that dead people are somehow coming back to life and seeking revenge on those who had wronged them. During their investigation, Jody asks to know why they are here and the brothers tell her to speak to their superior.

Unfortunately for Sam and Dean, Jody knows the man pretending to be their superior -- Bobby Singer. As Sheriff, Jody tells the brothers off and instructs them to leave the case alone. Sam and Dean soon find out that Jody is unwilling to deal with the mysterious resurrections because her son, Owen Mills, was among the people brought back from the dead.

When the resurrected start turning into zombies, Sam and Dean soon rush to Jody's house. They find Jody's zombified son eating from the remains of his own father, Sean Mills. Sam drags Jody from her house and kills her son for her. Afterwards, Jody helps the brothers kill the remaining zombies.

Season 7
In Slash Fiction, Jody comes to visit Bobby so she can repay him for saving her life earlier. Jody cooks and cleans for him, the latter which results in the cleaning product to drop onto a captured Leviathan Chet, whom Bobby has hidden in the basement.

Jody's actions reveal a Leviathan's weakness to Borax, a discovery which helps Sam and Dean in their fight against the Leviathans and Dick Roman.

Season 8
In Sacrifice, Jody goes on a date with a man named Roderick, with whom she shares a loss of loved ones. Jody becomes emotional over the memory of her son and husband and tries to calm down, only to start choking on her own blood.

Sam and Dean arrive at their location and reveal that the man Jody is dating is in fact a Crossroad Demon named Crowley. Crowley had targeted Jody among other allies of Sam and Dean in order to force them not to seal the Gates of Hell. As Jody lies dying, Crowley orders the brothers to say "I surrender" and at least one of them does, thus sparing Jody's life.

Jody is the only one of Sam and Dean's targeted allies to have survived Crowley's wrath.

Season 9
In Alex Annie Alexis Ann, Jody encounters a runaway girl known as "Alex" who is being hunted by vampires. Jody calls in Sam and Dean to help deal with the issue. With the brothers' help, Jody learns that "Alex" is in fact Annie Jones who was kidnapped eight years ago.

While Sam and Dean decide to track down the vampire nest, Jody does her best to protect Annie by keeping her at a family cabin. Annie refuses help however, and goes so far as to coldly talk about Jody's dead family. Despite Jody's best efforts, the vampires manage to capture Annie and take her back to their nest.

Sam and Dean arrive at the cabin the following morning to tell Jody that Annie had been used as bait to lure in unsuspecting humans for her adopted family to feed on. Jody decides to join Sam and Dean on their second attempt to take down the nest. At the location, Jody discovers the leader of the nest, Celia, who wants Annie to feed on Jody to complete her transformation into a vampire.

Because of Jody's kindness, Annie chooses to betray Celia and allow Jody to decapitate the vampire. Meanwhile, Sam and Dean take down the rest of the vampire nest. Sam and Dean use Celia's blood to cure Annie of her incomplete transformation. Jody then takes Annie back to the cabin and admits to the brothers that she wishes to keep Annie in order to fill the void caused by the loss of her own family.

Trivia

 * Prior to Mary's resurrection and after the death of Ellen Harvelle, Jody was the closest person Sam and Dean had to a mother. Her short-lived relationship with Bobby Singer (Sam and Dean's official adoptive father) also makes her an adoptive mother of sorts.
 * Alongside Donna Hanscum, Jody is one of the most recurring allies of Sam and Dean who are also law enforcers.