The public persona of a professional fighter is the face of relentless bravado. Even during the most charming of moments, a boast or a scowl are never far away.

Fighters are combustible and opinionated because that gets the job done.

But that posed a dilemma for Honolulu filmmaker James Sereno when fighter BJ Penn asked him to tell his story in the weeks before a historic bout with Canadian Georges St-Pierre.

Sereno knew there was a softer side to Penn, the darling son of Hilo, Hawai'i, and one of the most popular fighters in the history of mixed martial arts. After months of almost unlimited access, Sereno discovered a Penn rarely seen in public; the result is the filmmaker's first documentary, "BJ Penn 90 Days."

"Everybody thinks of BJ Penn the fighter," Sereno said. "I wanted them to know BJ Penn the man and his philosophy, his philosophy of life and how he trains and how he lives, what he believes in."

Sereno is known primarily for his work in local TV commercials and the short productions he made through Kinetic Films. He first met Penn while making a commercial.

A graduate of the University of Southern California School of Cinema/Television, Sereno's creative projects have always been fictional narratives that explored Hawai'i through the lens of culture. They were never about athletes; the 43-year-old filmmaker wasn't even a fan of mixed martial arts.

"I consider myself a local filmmaker and I like to tell local stories," said Sereno, who grew up in the Kaimuki-Kapahulu area. "I call it filmmaking from the inside out."

But Sereno knew Penn had a huge following — and he knew what made for a good story. This was a chance to set Penn's life story against the backdrop of a welterweight title fight. It was also a chance for Sereno to grow as a filmmaker.

"When you are doing a narrative or a commercial, you are manipulating all the action," he said. "In a documentary, you can't do that. It is unfolding. A documentary allows the truth to unfold. That was something I had to get used to."

Before Sereno would agree to do the project, he told the 30-year-old fighter that he had to be candid about his life. From his failings to his triumphs — from his fall to his resurrection, Sereno said — Penn had to be honest.

"I said you have to allow me to dig in and tell it all," Sereno said. "We are not going to sugarcoat it."

Penn wholeheartedly agreed.

Sereno started production 90 days before the title bout in January in Las Vegas. His crews followed Penn as he lived and trained in Hilo. It was a behind-the-scenes look: They were invited not only to workouts but also to live with family members — much of the Penn clan lives in the same neighborhood. Often, they began their day at the breakfast table.

Over the course of the 75-minute documentary, Sereno tells the story of how Penn grew up wanting to fight everyone and how his early and dramatic success in the ring led to cockiness, poor training and losses.

Sereno also found a man who learned from his mistakes and refused to give up. He said that Penn's honesty in front of the camera was surprising.

"There was a period when he had lost a couple of fights and he was not sure where to go next and what to do next," Sereno said. "He was very candid about where his mind was at, and I thought that was pretty incredible. And then he explained how he as a person sort of turned it around and how he was able to find that spark and that excitement."

Fans of mixed martial arts won't be disappointed, Sereno said. The documentary includes numerous fight scenes from Penn's career, including footage from one of his breakthrough victories — the Brazilian World Jiu-Jitsu Championship.

The bout with St-Pierre did not provide a storybook ending for Sereno. The Canadian battered Penn so fiercely that the fight was called after four rounds. But the documentary was never about about winning or losing, Sereno said.

"BJ's life is about facing challenges, attacking the challenges and moving forward and trying to rise above them," Sereno said. "He has won and he has lost. He has constantly moved and grown."

True to his word, Penn summoned Sereno to Hilo three days after his defeat in Vegas. Penn's face was still purple and swollen, but he faced the camera and talked about what happened.

He didn't make excuses. It was a side of the fighter Sereno always knew he would find.