Editor’s Note: This story was updated on Nov. 21.

A night out at the Borderline Bar & Grill was a rite of passage for young adults in the Thousand Oaks area. With country music, line dancing, cowboy boots and American flags, it was a place where college students and recent graduates came together with those who left high school planning to join the military or law enforcement. The Borderline was so beloved that its regular customers wanted to work there, earning money to help pay for first cars or college tuition.

This was the crowd that filled the Borderline on Nov. 7, when a gunman opened fire, killing 12 people.

Many of those who died were young, in their 20s. There was a college freshman and a graduate who had received his degree in May. There was the bar’s sunny cashier. And two friends who loved souping up old trucks for off-roading.

Among the others were a longtime sheriff’s deputy who had rushed into the crowded bar to help and a 22-year-old patron whose friends said he had tried to help others escape, yelling, “Everyone, run!”

If a thread binds the victims, it may have been their commitment to service. Several were active volunteers. One of the victims was a veteran, a 33-year-old who worked for an organization that helps veterans in their transitions to civilian life.

Three women and nine men were killed. The youngest victim was 18; the oldest was 54.

These are their stories.

Sean Adler

Image Sean Adler

Sean Adler, 48, had been a coach with a high school wrestling team in Simi Valley, Calif., but had recently changed careers. This year he opened a coffee shop called Rivalry Roasters, realizing a long-held dream, one of his childhood friends said. He was also holding down a job as a bouncer at the Borderline Bar & Grill, Royal High School’s wrestling team said.

Mr. Adler’s death was confirmed by his sister. Chris Curtis, an owner of Rivalry Roasters, said Mr. Adler had children and had been working at the bar to help pay the bills.

Danny Evans, one of Mr. Adler’s childhood best friends, said that although he had not spoken to Mr. Adler recently, he recalled that in high school, his friend “was the guy we all wanted to be: handsome, athletic and kind.”

“When we were younger, I was an awkward kid,” Mr. Evans said. “I got picked on.”

“And Sean was my protector,” he continued, choking back tears. “He stood up for me, and he showed me the kind of kindnesses I didn’t get from other people. I needed that so badly at that time.”

“I’m so proud to have been his friend,” he added. “It’s devastating that he’s gone.”

Blake Dingman and Jacob Dunham

Blake Dingman and Jacob Dunham, both 21, were close friends and off-roading enthusiasts who chronicled their adventures at local raceways on Instagram.

“Rest In Peace. Shred heaven,” one user wrote on Mr. Dunham’s last photo, posted June 8, from the Glen Helen Raceway in San Bernardino.

Mr. Dingman’s younger brother, Aidan, posted a photo of the two of them on Instagram, and described rushing to the bar with his parents when they got word of the shooting.

“We tried for hours and hours to get in touch with Blake and got no response,” he wrote. “At 12:00 this morning I was informed that my amazing brother was taken down by the shooter as well as his good friend Jake Dunham. Blake, I love you so much and I miss you more than you can imagine. #805strong.”

A local off-roading shop announced that it would hold a memorial for the two friends at a go-kart raceway in Thousand Oaks.

“I know we’re all heartbroken so let’s send them off in a way they’d want!” the shop wrote in an Instagram announcement.

Mr. Dunham’s mother, Kathy Dillon-Dunham, said that her son had faced health challenges, including hemophilia. But he always lived “full throttle,” she wrote in a Facebook message.

“He loved dirt biking which he started at age 7 on training wheels, jumping BMX bikes and everything else a child with hemophilia should not do,” she wrote.

“Jake was also the instigator of fun. If someone said that sounds like a bad idea, Jake would say ‘let’s do it!’”

Cody Gifford-Coffman

Image Cody Gifford-Coffman

Cody Gifford-Coffman, of Camarillo, Calif., had just turned 22 and was planning to join the Army, said his father, Jason Coffman, who confirmed his son’s death in an interview with reporters that aired on CNN.

A friend, Sarah DeSon, 19, said she believed she had survived the shooting because of Mr. Gifford-Coffman’s quick reaction. “He was protecting everyone,” Ms. DeSon said. “He got up and he just yelled, ‘Everyone, run!’”

Baseball was Mr. Gifford-Coffman’s passion. He played on his high school team and was an umpire. He liked spending time with his younger siblings and often went fishing with his father.

“I talked to him last night before he headed out the door,” Jason Coffman said in the interview outside the Thousand Oaks Teen Center, where families had gathered to await information. “First thing I said was, ‘Please don’t drink and drive.’ The last thing I said was, ‘Son, I love you.’”

Ronald Helus

Image Sgt. Ron Helus

Members of the Ventura County Sheriff’s Office said the death toll might have been higher if not for Sgt. Ronald Helus. As the shooting unfolded, Sergeant Helus ran inside.

“He went in to save lives, to save other people,” Sheriff Geoff Dean said. Before entering the bar, the sheriff said, Sergeant Helus had a conversation with his wife on the phone.

Sgt. Eric Buschow told CNN: “I don’t think there is anything more heroic than what he did.”

Sergeant Buschow described Sergeant Helus as “a true cop’s cop.” In 29 years on the force, he worked in various departments, including narcotics and SWAT, his colleague remembered.

“He had a natural instinct going after crooks,” Sergeant Buschow said. “He did it with enthusiasm and a great deal of intelligence.”

Sergeant Helus, 54, had been set to retire this year.

He loved being outdoors and fishing with his son in the Sierra Nevada. On his LinkedIn page, he said that in addition to his policing job, he owned a firearms safety training business called Gun Control.

Expressing condolences for those who died in the shooting, Gov. Jerry Brown singled out Sergeant Helus, who he said “took heroic action to save lives last night.”

The morning after the shooting, as Sergeant Helus’s body was taken from a hospital to the county medical examiner’s office, members of the sheriff’s office accompanied him in a solemn procession.

Alaina Housley

Image Alaina Housley

Alaina Housley, a freshman at Pepperdine University, loved music and soccer.

Ms. Housley, 18, of Napa, Calif., was a graduate of Vintage High School. She had written on Facebook that she planned to major in English literature. She also said she hoped to be accepted into a vocal music program.