Former Spirit (SAVE) - Get Report CEO Ben Baldanza may have left the stage for now, but the man who worked beside him for 14 years at two airlines is still very much engaged in shaking up the U.S. airline industry.

Barry Biffle, the president of privately held, Denver-based Frontier Airlines, worked closely with Baldanza at US Airways and then at Spirit. "I talked to him multiple times a day for 14 years," Biffle said Thursday. Together, the pair created the ultra-low-cost airline industry in the U.S: The carriers are now commonly referred to as ULCCs.

On Thursday, Frontier unveiled 42 new routes in 15 cities, including nine routes in American's Philadelphia hub and seven in Delta's Atlanta hub. Cities with new service include Austin, Charlotte, Cincinnati, Cleveland, Houston, Kansas City, Milwaukee, Nashville, New Orleans, Phoenix, Raleigh-Durham, St. Augustine and Seattle.

Competition from the majors doesn't worry Biffle. "We're looking at the markets that are the most overpriced and underserved," he said. "There are a lot of them." The approach explains why the ULCC segment is by far the fastest growing industy segment. Frontier's capacity grew 20% in 2015 and will do the same in 2016.

Biffle said he believes he was the first person to publicly use the term "ultra-low-cost carrier" when he mentioned it at an aviation conference in Miami in 2007. He and Baldanza were just getting going.

"We went to Spirit in 2005 and we didn't announce until a year later that we would be a ULCC," Biffle said. "Ben and I together had to intellectually learn that's where we wanted to go.

"We had worked at US Airways; we worked our hearts and souls out trying to fix it," he said. "We weren't ULCC people and there wasn't 'ULCCs for dummies' lying around. We had to study Ryanair, AirAsia and Tiger {Airways}, and learn what they were doing. We had to intellectually learn that's where we wanted to go."

One night, in the midst of the conceptualizing, the Baldanzas and the Biffles had dinner at the Biffle home in Miramar, Fla. The husbands asked their wives, "What do you think about charging for bags?" In response, Biffle recalled, "They both looked at us like have you lost your minds?"

Today, bag fees add billions in revenue for U.S. airlines. Spirit is the eighth-largest U.S. carrier by revenue and Frontier is 10th with revenue of $1.2 billion in the first three quarters, according to U.S. Department of Transportation statistics. Frontier's net income for the first three quarters was $114 million.

Biffle wouldn't comment on two widespread perceptions: that Frontier will go public and that Frontier and Spirit will merge. Both fly Airbus fleets. Also, Frontier is owned by Indigo Partners, once the principal owner of Spirit.

But Frontier and Spirit have their differences: Biffle said he puts more emphasis on pleasing passengers with better operational performance and the opportunity to pay for a higher service level.

According to DOT figures for on-time performance in the third quarter, Frontier ranked 11th among the 13 biggest airlines with a 78.7% on-time rate, while Spirit was 13th at 69.6%. Biffle said the carrier has improved its operational performance and is working on further improvements.

Additionally, Frontier courts more upscale passengers with an amenities package, called "the works," that includes a refundable ticket, seat selection, priority boarding, a carry-on bag and a checked bag. List price is $125 but right now the package sells for $49.

It's important to remember that the ULCC business is just getting started in the U.S. Biffle compares Frontier's present to the days when Southwest (LUV) - Get Report was ramping up in 1980s.

 "If you go back in history and you look at Southwest, they started out and took productivity, utilization and efficiency and brought costs down, which was a great thing for consumers," he said. "Then Ryan Air took it a step farther: They did all the efficiencies and also unbundled," charging fees for ancillary services such as checking bags.

"When I was a kid in Texas, everybody talked about Southwest the way people talk about ULCCs today: They didn't have seat assignments, they didn't serve meals and they had flight attendants in hot pants," Biffle said.

Now, Southwest is part of the airline establishment. It is Frontier and Spirit that are shaking things up.

This article is commentary by an independent contributor. At the time of publication, the author held no positions in the stocks mentioned.