SOUTH END -- Some called him "Bobby." Others, "BK." A few of his brother firefighters called him "The Natural," a nickname that, out of humility, he refused to accept. The Parkway Falcons Pop Warner youth football players called him Coach. Many called him a hero, a name he also rejected. His favorite thing to be called was "Dad."

Robert Kilduff Jr., a 24-year veteran of the Boston Fire Department, was killed battling a blaze on Treadway Road in Dorchester on May 23. The 53-year-old Marine veteran, a member of Rescue Company 2 in Egleston Square, followed his father and grandfather into the fire service. His death was the first line-of-duty death of a Boston firefighter in 12 years. He is survived by his girlfriend, Jess Spruell, and his children, Hanna Jane Kilduff, 24, and Mason Kilduff, 22.

Boston honored Kilduff at a June 1 funeral Mass at the Cathedral of the Holy Cross, celebrated by Father John Unni, chief chaplain of the Boston Fire Department, and presided over by Archbishop Richard G. Henning.

"He loved this city, he loved his community, and he loved his job," said Hanna Jane Kilduff in a remembrance before the Mass. "He used to say that the only thing he loved more than being a firefighter was being our dad."

She said her father "represents everything our world can be if we look out for each other, if we reach out to help first and ask questions later." Everyone in the cathedral, she said, probably had a story about him helping them.

"Every day, he embodied what it means to be a good neighbor, a caring friend, an empathetic stranger," she said. "Nobody was out of the bounds of his compassion."

Mason Kilduff called himself "a Boston guy," like his dad. The two joked that they bled green and stayed connected through calls about Boston sports, even when they were 700 miles apart.

"I was always so proud to call him my dad," he said. "He was the one front and center when it came to helping someone. All I thought when I saw that was, 'Yeah, that's my old man.'"

When Mason Kilduff was in Marine boot camp, his father sent him letters and hoped he would surpass his own accomplishments. Mason said he would be satisfied to become half the man his father was.

"My dad is my hero," he said. "He's the hero everyone says he is, and I'll miss him forever. And he taught us all how to live, and now he's taught us how to die."

Before the Mass began, mourners lined Washington Street outside the cathedral as hundreds of firefighters and other first responders in dress uniform stood beneath two fire truck ladders holding a large U.S. flag. A procession of police motorcycles, buses displaying "RIP Firefighter Robert Kilduff," and fire vehicles marked the solemn arrival.

The crowd, including Mayor Michelle Wu, Gov. Maura Healey, Sen. Elizabeth Warren, and other officials, stood in silence until hundreds of white-gloved firefighters and first responders raised their hands in salute.

A large contingent of bagpipers in black uniforms led the procession. Kilduff's casket, draped in the U.S. flag, was carried atop Rescue Company 2 Engine 42 and borne up the cathedral steps. Inside, mourners filed past in near silence as pews filled with first responders holding their caps to their chests.

In remembrances before the Mass, several speakers paid tribute to Kilduff.

Boston Fire Commissioner Rodney Marshall said firefighters have a "sixth sense" for recognizing the best of their profession.

"I could tell Bobby was one of those firefighters," he said.

Mayor Wu said that Kilduff's life represented the spirit of Boston -- "a simple act of faith that our people will show up for one another."

"Last Saturday night, Bobby Kilduff answered the call," she said. "He gave his life making sure his neighbors got another day."

Boston Firefighters IAFF Local 718 President Sam Dillon called Kilduff a brother and friend. He noted that the words "Losing a firefighter brings back every other loss," inscribed on the Hotel Vendome fire memorial on Commonwealth Avenue, resonated deeply at Kilduff's funeral.

"Losing a firefighter like Bobby Kilduff is devastating because no one embodied or believed in our principles, our cause, and our mission more than he did," Dillon said. "Not just a firefighter. A United States Marine, a humanitarian, and a true friend, and above all else, a father."

Firefighter Vic Gaybor of Rescue Company 2 said he worked with "BK" for 15 years. Their fellow firefighters joked that the two bickered like a married couple, and Kilduff was known for pranks. But when there was a fire, Gaybor said, he was all business and fully deserved the nickname "The Natural."

"BK was an exceptional firefighter, if not one of the best, the best," he said.

Greg Kelly of Rescue Company 2 said he was with Kilduff on his deathbed, along with Father Unni and Father John Currie. He said the priests and firefighters prayed with him and that Kilduff knew they were there in his final moments.

"He's leaving such a hole in this crew," Kelly said.

Kelly said Kilduff died doing what he loved. His last act before the fatal fall, Kelly said, was to warn that the ladders needed to be backed up because the building was unsafe.

Kelly told Kilduff's family to "weaponize" their faith and let it lead the way through their grief. He also prayed that the pain of Kilduff's loss would never be forgotten, and that it would strengthen the resolve of his fellow firefighters.

Father Unni began his homily by simply saying, "Where to even begin at a time like this?"

He told Hanna Jane and Mason Kilduff that "your dad, Bobby, BK, our friend, is okay."

He urged Hanna Jane and Mason Kilduff to place their grief and their questions about the afterlife into the Gospel proclaimed at the Mass, from John's account of Jesus saying that in his father's house there are many dwelling places.

Father Unni said Kilduff also created dwelling places, making people feel welcome, especially at the firehouse. Like everyone else, he said, Kilduff had been on a mission: he came from God and had returned to God.

"Your father, Bobby Kilduff, lived out his mission beautifully and well," Father Unni said.

Father Unni said no one was a stranger to Kilduff and that everyone could rely on him.

"He always felt better doing things for others," Father Unni said. "He was passionate, empathetic, humble, never taking credit, but was the guy who got it done."

Father Unni told the assembly that Kilduff's life should be an inspiration for them to live as he did. He quoted St. Paul: "Keep on doing what you've learned and accepted and heard and seen in me, do these. And so shall the God of peace be with you."

"That's BK," he said.

Like Kilduff, Father Unni said, the first responders in the cathedral continue their work despite the dangers. What they can control, he said, is holding their friends and families close, as Kilduff did.

"That's the healing balm for us at a time like this," he said.

After Mass, Kilduff's family stood before the altar as his girlfriend, Jess Spruell, thanked those gathered for their support and prayers.

"Seeing all of you here today reminds us just how much he was loved," she said.

She said the grief surrounding his death reflected not his titles or accomplishments, but the way he made people feel.

"He was the person who checked in, showed up, helped without being asked, and quietly carried burdens that weren't his to carry," she said.

He did not seek glory, she said. He just wanted to help.

"That was the man I loved," she said.

"If you want to understand who BK really was you only have to look at his children," she said, adding, "The incredible people they've become are part of his legacy."

Spruell said Kilduff's daughter once told him she loved Disney World because his phone did not ring there and he did not have to leave. From then on, he embraced all things Disney. Spruell also recalled that he once flew from Boston to Seattle to watch her daughter perform in a school play.

"Show up for one another," Spruell told the assembly. "Laugh together. Love people the way he did."

Kilduff's family received the Martin E. Pierce Medal of Honor, awarded to the families of firefighters who die in the line of duty. As it was presented, the cathedral fell silent except for sobs. Archbishop Henning then offered special remarks to the firefighters inside the church and outside.

"We trust in you, and I'm grateful for the sacrifices you make," said the archbishop, himself the son of a firefighter. "And we never fail to pray for you and your families making that sacrifice."

Above all, he said he was grateful to Kilduff's family.

"I think it's fair to say that in a very real sense, you shaped him as well," he said. "He knew what he was fighting for. It was ever and always you."

The archbishop incensed the casket as a choir member sang the Irish blessing, "May the road rise to meet you... ."

A silver bell was rung in Kilduff's honor, following a centuries-old tradition marking the end of a firefighter's service. Then the cathedral bells sounded. First responders, dignitaries, mourners, and clergy poured outside beneath clearing skies as bagpipers played "Amazing Grace."

Kilduff's casket was returned to Engine 42 and the procession moved out through a final wall of salutes, carrying him to Rescue Company 2 one last time and then to his final rest.