The John Legere of the hot pink T-shirts and modish hair? That, he said, is “the real me. I’m totally comfortable with it.” He said his father wore conservative Brooks Brothers suits and button-down shirts, which he emulated earlier in his career. But now he has reverted to the jeans and longer hair of his college days, when he was a cross-country star at the University of Massachusetts. Next week, he’ll be in Washington meeting senators and members of the House, and he said he would be wearing his signature T-shirt. “I may tone down the sneakers a little bit and wear a suit over it,” he said. “That’s still under negotiation.”

Beneath the clothes is a fierce competitor who still runs marathons (including this year’s in Boston) and seems to enjoy nothing more than denouncing AT&T and Verizon, which he refers to as a “pseudo duopoly.” (He doesn’t think Sprint is even worth mentioning.)

When he joined T-Mobile, “We had a limited time window and a sense of urgency,” he told me. “We were losing over two million customers a year. So we moved as fast as we humanly could. My board wondered if we were doing too much. But the fact is, speed has become one of our biggest weapons. The current industry is arrogant, stupid and slow, which gives companies like T-Mobile a real competitive advantage.”

Though the task of reviving T-Mobile seemed daunting, “I felt that as a challenger, we might just have a pretty amazing opportunity in front of us,” he continued. “We could take a completely different approach to this business, we could create real customer value by driving serious change in this ridiculous and broken industry.”

That meant upending long-entrenched industry practices, addressing what Mr. Legere calls customer “pain points.” T-Mobile got rid of the much-reviled two-year contract, and let customers pay for their phones and service separately. If they don’t like their T-Mobile service, they can change carriers any time once their phones are paid for. T-Mobile cut prices, with free unlimited overseas roaming. It offered customers the iPhone and other popular models and allows them to upgrade their phones every six months. Armed with a war chest and additional spectrum that it got as a breakup fee after the AT&T deal failed, T-Mobile expanded and modernized its 4G LTE network, now available to 205 million people. A few weeks ago T-Mobile roiled the industry again when it offered 200MB of free data monthly for tablets.

Representatives of AT&T and Verizon declined to comment.

T-Mobile branded and marketed all this as the “Un-carrier,” rolling out new versions of its plans — already five and counting — even as competitors have struggled to match the previous one. “Surprise is an effective competitive tactic,” Mr. Legere said. “When you catch the competition by surprise, keep punching and don’t let them up. When you have momentum, keep building it by delivering unexpected offers in rapid succession. Our team is loving it. They are breaking the boundaries and getting to use all their creativity. That’s pretty cool.”

Another group that’s loving it is antitrust regulators. T-Mobile’s success has enabled the Justice Department’s antitrust division to take a rare victory lap, since the company’s consumer-friendly moves are exactly what regulators were hoping for when they sued to block the merger with AT&T. “This really demonstrates that competition can work,” said William J. Baer, the current head of the antitrust division. “When you have feisty rivals whose survival depends on innovating and differentiating, they can gain market share and loosen the oligopoly. That’s exactly what T-Mobile has done.”

Whether T-Mobile can sustain the momentum remains to be seen, and Mr. Legere concedes the battle is far from over. “We run scared every day,” he said. “We are playing this game for the long run, and we know we have a lot to do yet. But look at the choices customers are making. In the last two quarters, since we launched Un-carrier, T-Mobile has grown faster than everyone else, and we have added more postpaid customers than Sprint, AT&T and Verizon combined. I really think that says it all.”