As a dentist, Seunggun Lee was used to filling holes in people's teeth. But when he spotted a gap in South Korea's banking industry, it was too blatant to ignore.
It didn't matter to him that the idea was illegal at that time.
"I thought this service should exist in Korean society — regardless of whether it was legal or not," Lee told CNBC Make It.
The young entrepreneur was hardly risk-averse, after all. Something about quitting his steady job as a dentist and pumping $400,000 (a 50/50 combination of his own savings and bank loans) into a "bunch of crazy ideas" gave him a certain brazenness.
He started the business in 2014, when similar peer-to-peer (P2P) money transfer apps — such as PayPal's Venmo — were taking off internationally. But the red tape in South Korea's famously regulated banking system made such services effectively illegal.
"I was able to see a lot of products that existed in the U.S. and I thought: If this was available in Korea, then it's going to be huge," said Lee, now 37, who described the country's transfer system as a "cumbersome" multi-step process at the time.
But getting it off the ground took some convincing — not just of regulators and investors, but his parents too.
Lee spent a year working with regulators to win them over to his idea for a simplified money transfer platform.
"They were like 'Oh, this is new,' so they just closed it," he told Quartz of the regulators. But eventually, in 2015, they agreed to ease the law on money apps and Toss launched in earnest.
With his parents, however, it was much harder.
When he told them he was packing in his career as a dentist to become an entrepreneur, they were furious. The uncertainty of entrepreneurship can be a difficult pill for parents to swallow at the best of times. But in South Korea, where conservative values persist, it can be even harder.
"It wasn't pretty," recalled Lee. "It really tore my parents down. In Asia, in general, there's this expectation from family and society that you'll do something traditional like be a doctor or a dentist. But I was never that guy."
Their hesitance was not without justification. When he initially quit his job, he spent more than four years and his life savings experimenting with many ideas. Eight start-up failures later — which ranged from social media networks to mobile apps — he found success in Toss.
But Lee was determined to "make an impact on the world" — with or without his parents' support.
"I spent three months trying to persuade them (to let me quit), but was unsuccessful. Eventually, I just gave up trying," he said.
That determination eventually paid off. Investors quickly saw value in Toss, and began pumping in funds to support Lee and his growing tech team.
"We saw a large opportunity and a trend that was happening outside of Korea," Moonsuk Oh, principal at Altos Ventures, told CNBC Make It. "And we saw a founder that had the vision and the commitment to execute on that."
A $80 million funding round in December 2018 took Toss over the $1 billion threshold and made it one of several South Korean unicorn start-ups. In August this year, total funding raised jumped to $261.5 million, which gave Toss a valuation of $2.2 billion.
That demand has extended to users too. In less than five years, Toss has registered 14 million registered users — 27% of South Korea's 51 million-strong population — and has processed over $48 billion in payments, according to the company.
"My instinct and my use test showed me that everyone needs to do money transfers for various reasons," said Lee. "I thought that could be millennials but also many other groups."
According to EY fintech analyst Varun Mittal, Lee spotted the opportunity at just the right time, when Korea's banking scene was ripe for innovation.
"People wanted a better way to (settle debts) than remembering bank account numbers," Mittal, EY's global emerging markets Fintech leader, told CNBC Make It.
"(Using) phone numbers is much easier for both sides — the lender and the borrower," he noted.
Today, South Korea is home to some 400 fintech companies. That's a number Mittal said is only going to rise as more consumers look for simplified banking solutions.
"It's only going to grow because the number of online transactions people make are going to grow. As long as the economy continues to grow, they will grow," said Mittal, who defined that growth as expansion in user numbers, transactions, and overall dollar value.
For his part, Lee said he plans to capitalize on that demand by further expanding Toss' offering in South Korea. In April 2019, the start-up launched a Toss Card for online and offline use, adding to its already broad range of services which include credit score management, savings accounts and insurance plans.
"Half (of our users) are coming not for P2P money transfer, but for other reasons," said Lee, noting that the app is set to hit 10 million monthly active users in September. "It's a financial platform as a whole. We want to include any financial needs for users."
"Our obvious expansion plan is towards Southeast Asia. Other markets have already been dominated by other services," said Lee, citing Alipay and WeChat Pay in China, Venmo in the U.S. and Revolut in Europe.
"The only other market is Southeast Asia," said Lee.
And with more than half the adults in Southeast Asia thought to be unbanked, that's some gap to fill. But fortunately Lee's parents are now fully behind him.
"They're full of pride these days," Lee said.