Alex Payne, who was one of Twitter’s early engineers and was the lead on its application programming interface, has left the company to co-found a financial startup, BankSimple. He’ll be the company’s chief product and technology officer and work out of Portland, Ore., although the company’s co-founders are in New York.

Payne worked on a critical part of Twitter, its application programming interface or API. The API effectively made the company what it is today; without opening its data up for use by others, Twitter wouldn’t have been able to foster an entire ecosystem of 100,000 applications. His last position at the company was as an infrastructure engineer, according to spokesperson Sean Garrett.

He also became well-known for his writing and general outspokenness. A controversial tweet a few months ago even landed him in hot water with developers. He said that Twitter was set to launch features that would make the company’s site better than readers from third-party startups like Tweetdeck and Brizzly. Following the snafu, he pledged to lay off blogging.

Payne announced the move on his blog:

“Walking away from Twitter wasn’t an easy decision. Working there has been a life- and career-changing experience. I’ve learned all sorts of lessons, made great friends, and worked on something that millions of people now use every day,” he wrote. “But millions of people also need a better bank. It’s a different sort of problem than Twitter, and it’s going to be a very different sort of company. But I feel ready to take on the challenge that BankSimple presents, and I’m joining on with two brilliant people who’ve put a ton of thought into building a better bank.”

Payne’s new venture calls itself a “social bank” and says it doesn’t have hidden fees or charge for overdrafts. The two other founders Josh Reich and Shamir Karkal ran a boutique data mining consulting firm and consulted for banks internationally as part of McKinsey respectively. It’s unknown whether the company has angel or venture funding at this point. The company did not respond to immediate requests for comment.