SAN FRANCISCO — Vlad Tenev, the chief executive of the online brokerage Robinhood, has had practice doing damage control.

Last March, he told customers that “we owe it to you to do better” after Robinhood’s app suffered lengthy outages, leaving many people unable to trade.

In June, he wrote in a blog post that he was “personally devastated” and wanted to improve the “customer experience” after a 20-year-old who had a negative $730,000 balance on the app killed himself.

And in December, when federal regulators fined his company $65 million for misleading users about how it made money, he said the accusations “don’t reflect Robinhood today.”

Mr. Tenev, 33, is now in the hot seat again after Robinhood abruptly curtailed its customers’ trading last week amid a frenzy in stocks such as GameStop, which were driven sky high by an army of online investors. The limits infuriated Robinhood’s users, who were locked out of the action, and the seven-year-old start-up was blasted by lawmakers and others, accused of acting unfairly toward ordinary investors.

For days, Robinhood was slow to fully explain why it had curbed people from trading the stocks. Only later did Mr. Tenev disclose that Robinhood had put in restrictions because it did not have enough of a cash cushion to hedge against the risky trades. To increase that cushion and avoid further problems, Robinhood raised an emergency $1 billion last week, followed by an additional $2.4 billion this week.

On Sunday, Mr. Tenev told Elon Musk in an impromptu interview on the online conversation app Clubhouse that he knew that Robinhood’s trading curbs were “a bad outcome for customers.” He said the entire experience had been challenging, “but we had no choice in this case.”

It was no surprise that Robinhood got caught unawares over the past week, current and former Robinhood employees and analysts said. While Mr. Tenev has helped revolutionize online trading for a younger generation with an app that makes investing easy and fun, his start-up has repeatedly been ill prepared to deal with issues as commonplace as technology glitches and trading hiccups, they said.

Many start-ups go through growing pains. But “there’s a consistent pattern which makes one question whether he knows what is going on inside his company,” Vijay Raghavan, an analyst at Forrester Research who covers Robinhood and other brokers, said of Mr. Tenev.

Lawmakers and some of Robinhood’s users have been even harsher on the chief executive. Representative Alexandria Ocasio-Cortez, Democrat of New York, and Senator Ted Cruz, Republican of Texas, have slammed Robinhood for freezing users’ ability to buy GameStop stock. Mr. Tenev has agreed to testify about the issue in Congress on Feb. 18.

Even some of Robinhood’s biggest promoters have turned against Mr. Tenev. Dave Portnoy, the founder of Barstool Sports and a high-profile Robinhood supporter, wrote over a picture of Mr. Tenev on Twitter last week: “Fraud, liar, Scumbag.”

Robinhood, a privately held company in Menlo Park, Calif., declined to make Mr. Tenev available for an interview. But Jason Warnick, the chief financial officer, said Mr. Tenev had widespread support internally.

“When I watched Vlad, there is absolutely no one else I would want to be with,” Mr. Warnick said about the events of the last week. “He mobilized us in an incredibly effective way.”

Venture capitalists who have backed Robinhood, which is valued at nearly $12 billion and is likely to go public this year, also said they had confidence in Mr. Tenev. Rahul Mehta, a partner at the venture firm DST Global, said the speed with which Mr. Tenev had raised the emergency $3.4 billion over the past few days “shows you the support around the table and the belief people have, in particular, for Vlad.”

Mr. Tenev, who moved to the United States from Bulgaria when he was 5 and grew up in the Washington, D.C., area, founded Robinhood with Baiju Bhatt in 2013. The two met while studying math at Stanford University.

After graduating from Stanford in 2008, Mr. Tenev attended the University of California, Los Angeles, to pursue a Ph.D. in math but dropped out to work with Mr. Bhatt. The pair initially had two other business ventures, including a Wall Street trading firm.

But those were short-lived. Instead, inspired by the Occupy Wall Street movement in 2011 — which took aim at the power of the big banks — they began talking about how to “democratize finance” for everyone by ending the fees that most brokerages charged to trade stocks. They named Robinhood after the English outlaw of legend who stole from the rich and gave to the poor.

In particular, Mr. Tenev and Mr. Bhatt wanted an app that a younger generation could easily use. “People in my age group, the millennials, weren’t getting into the markets and were openly distrustful of the institutions that were providing financial services,” Mr. Tenev said on CNBC in 2015.

Mr. Tenev and Mr. Bhatt, who were co-chief executives, made Robinhood simple. Users were able to begin trading stocks with nothing more than an iPhone app and with no fees. The app also made trading feel like a game. New customers were given a free share of stock after scratching off what looked like a digital version of a lottery ticket.

The men sought out celebrity investors like the actor Jared Leto and the rapper Snoop Dogg. The co-C.E.O.s often showed up at the office with matching Tesla sport utility vehicles, one black and one white, two former employees said.

Inside Robinhood, Mr. Tenev was known as the cerebral coder in charge of operations, said six current and former employees, who spoke on the condition of anonymity. He was known for sitting down at lunch with employees to talk about books or his latest theories from science fiction. Mr. Bhatt was more fun-loving and handled design, they said.

Both were active on social media, with Mr. Tenev tweeting emoji-filled, jokey replies to Mr. Musk. Mr. Bhatt broadcast pictures of themselves from floor seats at Golden State Warriors basketball games.

As Robinhood grew quickly, though, so did the blunders. In 2018, the company announced that it would begin offering bank accounts. But it had not secured approval from financial regulators, which is standard practice, earning the start-up a swift rebuke.

That same week, Robinhood released software that erroneously reversed the direction of customer trades, which meant that a bet on a stock going up was turned into a bet that it would go down. Mr. Tenev oversaw technology.

Technological issues continued piling up. In 2019, customers discovered that Robinhood’s software accidentally allowed them to borrow almost infinite amounts of money to multiply their stock bets. Last March, as the pandemic hit the United States and the stock market gyrated wildly, Robinhood’s app seized up for almost two days, leading some customers to lose more than $1 million.

That was when Mr. Tenev said in a blog post that “we owe it to you to do better.” By then, Robinhood had more than 13 million customers.

Mr. Warnick and other employees said Mr. Tenev had a knack for staying calm during difficult situations. “He doesn’t get emotional,” Mr. Warnick said.

But five current and former Robinhood employees said Mr. Tenev moved quickly to new projects without fixing the previous problems. After the March outages, they said, Mr. Tenev told the company that it would significantly ramp up its infrastructure and customer support. Yet almost a year later, the start-up does not offer a customer service phone number, unlike its competitors.

Robinhood did not respond to a request for comment on the customer service issues.

Last year, Mr. Bhatt stepped down as co-chief executive after returning from paternity leave, leaving Mr. Tenev in charge. Mr. Bhatt remains an executive and is on the company’s board of directors.

Robinhood’s technical outages have continued. Last month, the site went down 19 times, more than twice as often as Charles Schwab or Fidelity, according to data from the web tracking company DownDetector.

Mr. Tenev has recently kept a low profile. Last year, he said in a podcast interview that he keeps his phone out of his bedroom at night to avoid being tempted to check social media.

But over the last week, as the mania over GameStop stock grew and Robinhood was forced to react, Mr. Tenev had little choice but to step out more. He has appeared on television at least eight times from the sparsely decorated living room of the home where he lives with his wife and children.

In most of the appearances, Mr. Tenev used technical language and shifted quickly to talk about Robinhood’s moving forward to another stage of expansion.

“This is just a standard part of practices in the brokerage industry,” he told Yahoo Finance last Friday, referring to the decision to temporarily halt some purchases. “We’re very confident about our future.”

Kitty Bennett contributed research.