When a team at U.S. Bancorp set out to design the bank's new mobile app, which launched last October, its members had a woman named Leslie in mind.

Leslie was a fiction they had dreamed up, a representation of the archetypal millennial, loaded with millennial traits and millennial desires. Millennials, they knew, would be the top users of the mobile app, and U.S. Bank very much wanted to cater to their needs.

For starters, millennials "like things done instantly," said Gareth Gaston, executive vice president of omnichannel at the $429 billion-asset U.S. Bank. So the mobile team created a Venmo-type peer-to-peer payment service within the app that lets a customer send small amounts of money to a friend — with whom they may be splitting a dinner bill — in a matter of seconds.

What's more, "millennials expect you to know who they are and to address them personally," Gaston said. So, on the U.S. Bank mobile app, the user can select a self-portrait to be greeted with upon signing in. The app also shows the user's last login date and time, which functions both as a security feature and as a way to show that the bank knows its customers.

Millennials are the fastest-growing customer segment for banks, and financial institutions of all sizes are working hard to attract and retain their business. That means investing in technology: mobile check deposit, peer-to-peer payment services, features like fingerprint recognition and mobile and online banking tools that speed up and take the pain out of day-to-day banking transactions.

For big banks, at least, the efforts seem to be paying off. Thanks to their deep pockets and, for some, an even deeper desire to come back from the blows their public image suffered during the financial crisis, they are leading the way in appealing to a youthful demographic.

At the $157 billion-asset Ally Financial, which has retooled its marketing to appeal to young adults, more than one-third of its customers are now millennials, outnumbering all other age groups. U.S. Bank did not disclose how many of its customers are millennials, but the company said that mobile is by far its fastest-growing channel and that mobile users are, on average, 10 years younger than online users.

Indeed, the big surprise in J.D. Power & Associates' most recent customer satisfaction survey is that the six largest banks are now outperforming midsize and regional banks when it comes to keeping customers happy. It marked the first time in the 11-year history of the survey that the largest banks — JPMorgan Chase, Bank of America, Wells Fargo, Citigroup, U.S. Bank and PNC Financial Services Group – scored higher than their smaller and regional counterparts.

The key differentiator is technology. While consumers say smaller institutions deliver better face-to-face service and charge lower fees, young consumers in particular say large banks are winning them over with more user-friendly mobile apps, websites and ATMs.

"They're doing extraordinarily well with millennials because of mobile, because of online," said Paul McAdam, senior director of banking services at J.D. Power.

The popular narrative says that millennials as a whole are big bank-hating Bernie Sanders voters, but the truth is that they value convenience, speed and responsiveness above all. The big banks are delivering in spades. In January, Bank of America announced that it would triple annual spending on its mobile app. At U.S. Bank, a team of 75 to 100 people is dedicated to continually improving the digital experience for customers.

"We are investing more in digital than other banks are able to invest, and with that we're providing a better digital experience," Gaston said. "I do think it's as simple as that."

Most small banks offer mobile banking as well, but they simply aren't investing as heavily in the technology as large banks are, experts said. Mobile check deposit is offered at all large banks and most regional ones, but the service is far from widespread at community banks.

Meanwhile, the biggest banks continue to add features in response to customers' demands. In March, U.S. Bank, based in Minneapolis, made it possible to log into its mobile app using one's fingerprint.

"Millennials are very vocal," Gaston said. Every day, he and his team check customer feedback using a survey tool called ForeSee; its analytics dashboard provides daily scores and lets them stay abreast of customers' complaints and desires.

Another bank doing well with young adults is Detroit-based Ally, in part because it has seen through another popular misconception about millennials: that they are poor. In fact, there are plenty of affluent millennials. "The millennial segment is huge. All millennials are not created equal," said Diane Morais, the chief executive at Ally's bank subsidiary, Ally Bank.

The bank, which exists solely online, is making a special effort to target young people with annual incomes of $100,000 or more. Many of them saw their parents suffer in the economic downturn, making them "very, very savings-conscious" even in an era of rock-bottom interest rates, Morais said. That, in turn, makes them ideal customers.

Ally is actively courting them, pivoting its marketing strategy in the first quarter of 2016 to find millennials where they are consuming media. That means broadening its outreach, placing online video advertisements on digital properties known to draw young people in droves. And the bank has shot new television ads featuring people and situations millennials can relate to.

Like other big banks, Ally is developing brand-new products and services to please younger customers. In July 2015, the bank added a "digital assistant" to its mobile app — a bot that helps users find the information they are looking for. You text or speak aloud to the bot — for instance, "How much interest have I earned this year?" — and it responds in written text.

Millennials "want information at their fingertips. They want it quick and easy, and they don't want to take time to hunt around for it," Morais said. Ally's hope is that, as the technology improves, the assistant will eventually be able to speak back, Morais added.

Roughly half the 53,000 new depositors Ally added in the first quarter were millennials. The bank has approximately 1.1 million deposit customers, 36% of whom are millennials with the remainder split evenly between baby boomers and members of Generation X.

Another reason banks are so eager to attract millennials now is it's a generation that stands to inherit unprecedented wealth; more than $30 trillion in assets is expected to change hands from baby boomers to their children and grandchildren over the next 40 years, according to Accenture.

"There's a long-held belief in banking that it's important to attract younger customers so they can grow with you, and we want to grow with them," Gaston at U.S. Bank said.

That potential transfer of wealth is a big reason Ally is plunking down $275 million to acquire TradeKing Group, an online brokerage firm. That deal, which was announced last month and is expected to close in the third quarter, will enable the bank to funnel its deposit customers into self-directed trading and guided wealth management services, said Michael Baresich, Ally's chief information officer.

"As their wealth grows, we grow, and we succeed along with them," he said.

Still, it's not all about the technology for millennials. They do still drop by branches on occasion — an average of 10 times a year according to some studies — and that presents opportunities for banks of all types to strengthen ties to this demographic.

"Sure, that's certainly much less than that number would have been 10 years ago," J.D. Power's McAdam said. "But there are plenty of retailers out there that would love to have a customer walk into their store 10 times a year."