If anyone understands online payments, it's Max Levchin.

The Ukrainian-born entrepreneur minted his reputation as something of a Silicon Valley superstar after co-founding the online payments service PayPal in the late 1990s. Levchin served as PayPal's chief technology officer for four years, during which time he built the complex software that kept the Website operating while staffing up the engineering team. All the effort paid off big when eBay EBAY -1.09% acquired PayPal for a cool $1.5 billion seven years ago (He reportedly banked $34 million.) Afterwards, Levchin had five-year run heading up mobile game maker Slide before selling it to Google, where he briefly served as vice president of engineering.

But for the last two years, Levchin has tried reinventing online payments, much the same way he did with PayPal. His latest start-up, Affirm, tackles consumer loans, but with a twist.