"Subprime" is the credit industry's degrading term for "below average" credit. The cost of being subprime is real: if your credit score is 500, you're paying up to 6x the interest rate of someone with a 750. Any lower and you're forgotten altogether; you live a precarious paycheck-to-paycheck life. If the price of gas goes up, your paycheck runs short. You can't afford your electric bill. You hope they'll let it slide a day or two, but come home to a dark house with food quickly going bad. Without the benefit of the credit safety net the rest of us take for granted, it's hard to get ahead.
Mengxi Lu, a software engineering manager in San Francisco, certainly took it for granted when building out a billing system for the first time. To process credit cards, he used a then-new API called Stripe, which kicked off a fascination with fintech that outlasted that first startup. When shut down came, he dove into researching how people build wealth.