A few years ago, Kevin Paffrath, a 29-year-old real-estate broker and father of two who lives in Ventura, Calif., earned most of his income from commissions on home sales, and kept up a side gig recording online videos about home buying.

Today, he’s MeetKevin, a YouTube influencer with 1.7 million subscribers. Most days, he live-streams on the platform for several hours, talking about the stock market and doling out investing advice in a rapid-fire, self-deprecating manner. He banters with commenters, hams it up with silly British accents and sips coffee in front of a wall in his home studio hung with brightly colored cartoon posters and a turquoise electric guitar. In addition to the live streams, he’s made hundreds of videos on a range of investment-advice topics.

“I used to spend three or four hours with one client talking to them about real estate they could buy for below market value, and if I was lucky, maybe after 90 days, that one person would turn into $10,000 dollars in revenue,” Mr. Paffrath said. “Today, $10,000 in revenue is a bad day for me. Influencing has crowded everything else out.”

Mr. Paffrath says he earned $5 million in the first three months of this year, as page views and demand for his guidance have skyrocketed during the pandemic. Receipts he provided to The Wall Street Journal showing YouTube revenue payments confirm that he earns several million dollars a year in ad revenue alone.

As the U.S. retreated amid the pandemic to its couches, millions of would-be stock pickers—some flush with stimulus cash—fired up social-media and messaging apps and dove headlong into the world of retail investing.

Total users at the six top online brokerages, which are used mainly by individual investors, topped 100 million in 2021. At Robinhood Financial LLC, the investing app popular among younger investors, accounts have grown explosively, from 7.2 million in March of 2020 to 18 million a year later, according to company financial filings.

Along with the rise of commission-free online trading has come demand for advice at the lowest price in the most accessible place: free, and online. Now, a new generation of Jim Cramers has risen up on social media with massive followings as guides to these market newbies.

Mr. Paffrath, aka MeetKevin, spends several hours a day live-streaming on YouTube from his home studio in Ventura, Calif.

Photo: Michael Friberg for The Wall Street Journal

Many of these influencers have no formal training as financial advisers and no background in professional investing, leading them to pick stocks based on the whims of popular opinion or to dispense money-losing advice.

In addition to MeetKevin, popular finance influencers include Marko Zlatic, who under the name Whiteboard Finance gives advice to 670,000 subscribers on everything from buying a used car to building an investment portfolio based on ideas from hedge-fund manager Ray Dalio.

Some influencers with smaller followings are more colorful. Jack Spencer, a former personal trainer in Ireland, traded fitness-tip videos for stock picking in March of 2020. Mr. Spencer, 23 years old, interviews startup CEOs wearing a shirt that reads “Not A Financial Advisor,” with a view of his well-stocked liquor cabinet in the background. Mr. Spencer, who has 94,000 YouTube subscribers, didn’t respond to requests for comment.

On TikTok, podcaster Tori Dunlap (known on the app as @herfirst100k) offers her 1.7 million followers tips for developing “side hustle” businesses and which debts to pay off first. The 27-year-old is developing an app to cultivate a “nonjudgmental investing community” intended to help women feel comfortable trading investment tips and managing investments in their brokerage accounts.

Cameron Newell, aka CamTheMan, a college dropout from central Washington state, started trading penny stocks full-time about three years ago and says he earned $5 million last year from day trading. He doles out stock tips on TikTok and hosts a chat group on Discord—the social app commonly used by videogame enthusiasts—where followers can track and copy his trades, or follow along with periodic investing challenges where he seeks to turn an initial investment of $1,000 into $1 million.

“Traditional finance is a black box,” said Sarah Petite, a social-media consultant in Los Angeles. “This generation is looking at their parents and saying, ‘The way you thought about money? That isn’t how it works anymore.’ ”

Instead, young people want a road map to how to make big profits, and a strong independent streak draws many of them to online influencers who dole out advice free, rather than paying a traditional investment manager to handle their money for them. Many don’t care much about the qualifications of who’s giving the advice, experts and young investors themselves say.

“I have a rule: Don’t pay for something you can get free,” said Rex Wu, a 33-year-old investor from Tampa Bay, Fla., who is a regular viewer of Mr. Paffrath and several other online finance-world personalities. He says he has invested a few hundred thousand dollars based on “things I’ve learned online from guys like Kevin.” So far this year, his portfolio has returned 23%; year-to-date, the S&P 500 has returned 21%.

Like other finance influencers, Mr. Paffrath has learned the hard way that skeptical assessments of trendy stocks can cost him subscribers.

Photo: Michael Friberg for The Wall Street Journal

“If I were to walk into J.P. Morgan tomorrow, they have the bias of trying to earn my business, and they might be trying to oversell me,” Mr. Wu said. “Guys online don’t really have anything they’re trying to sell me.”

Still, the online model is entirely new, with influencers judged on the content they produce, rather than their investing track records; and they often get paid based on numbers of subscribers and viewers, rather on the investment income their advice generates for clients.

Rule 1: Be relatable

This new breed of social-media finance gurus has tapped into young people’s innate distrust of the financial establishment, which they see as controlled by Wall Street insiders who come from an older generation to whom they can’t relate.

“No one my age is watching cable TV anymore,” said Ms. Petite, who is 24. “I don’t know a single person who regularly watches TV financial news. They’re going to these specialized YouTube channels and social media. Giving their money to some guy in a suit on Wall Street, I just don’t think that’s something that would ever even occur to them.”

Building trust is a key part of the YouTube stock-calling business.

Rose Han, a 32-year-old with a finance degree from New York University, quit her job as a currency trader at HSBC five years ago and started making YouTube videos about options trading, fund investing and personal financial accounts.

‘If you turn on CNBC, it’s all these older white guys, and it’s hard to relate to them,’ said 32-year-old influencer Rose Han.

Photo: Jake Naughton for The Wall Street Journal

She has half a million YouTube subscribers and 54,000 followers on Instagram. She says she now makes 10 times as much as she made as a trader selling online investing courses.

“If you turn on CNBC, it’s all these older white guys, and it’s hard to relate to them because I don’t look like them; a lot of my followers don’t look like them,” Ms. Han said. “Being a woman, women trust me more, because they’d rather learn from someone like me than from a finance bro.” She says she has made $2 million so far this year from a combination of YouTube ads and sales of her own online finance courses.

Rule 2: Sell the dream

Generation Z—roughly the cohort born between 1992 and 2002—is more likely to respond warmly to cultural signifiers of wealth like fancy cars and high-fashion clothes than they are to business degrees, financial certifications or a perch on a TV-news show, experts say. For that reason, there is big potential for fraud and financial ruin, said Ted Klontz, a professor of behavioral finance at Creighton University.

“A lot of this world is a real snake pit, and it’s really scary,” Mr. Klontz said. “The human brain doesn’t come into full maturation until age 25. Anyone under that age is highly influenceable. We know that day trading does not produce long-term wealth for the vast majority of people who do it, but these influencers are preying on that part of the human brain that has fewer inhibitions, that thinks: ‘I will be the exception.’ And that leads to speculation and other kinds of very high-risk behavior.”

TikTok is filled with promotional accounts like the Daily Trader, a self-professed 19-year-old millionaire who posts videos of himself driving a brand-new Audi R8 sports car, lounging by the pool in what he describes as a $20 million Beverly Hills mansion and eating at Los Angeles’ priciest steakhouses.

These glitzy TikTok videos link to his profile on Discord, where he sells memberships to his “mentorship group,” which includes a video tutorial series to help day traders emulate his “trading system,” for $294 each. At least 1,600 users have signed up for the course, according to information shared with a Journal reporter after joining the group. The Daily Trader didn’t respond to requests for comment.

This aspirational dynamic creates pressure for creators to deliver only the positive stories that their viewers want, creators say, which can sometimes turn out to be spectacularly off the mark.

Casey Adams, a 20-year-old who grew up near Richmond, Va., has conducted 300 interviews with executives and investors for his YouTube channel, including with Netflix Inc. co-founder Marc Randolph, celebrity startup founder and cryptocurrency investor Tyler Winklevoss and at several points over the past year, Maye Musk, mother of Elon.

Casey Adams favors softball questions in his YouTube interviews. ‘My approach is a friendly approach,’ he says.

Photo: Michael Friberg for The Wall Street Journal

He earns about $300,000 a year from ad placements and his own media-consulting business, he says, but recognizes that many of the billionaires and startup founders who agree to come on his show are there for the softball questions and the chance to speak directly to young individual investors.

Last year, Mr. Adams scored a rare half-hour interview with Trevor Milton, the billionaire chief executive of electric-truck startup Nikola Corp.

“You guys are not only crushing it, but you’re paving the way for a completely new industry,” the baby-faced Mr. Adams gushed to Mr. Milton at one point in the interview.

Four months later, after Nikola had gone public in a reverse merger, a short seller released a damning report accusing Mr. Milton of improprieties, the startup’s share price fell by more than two-thirds, federal investigators opened a probe into the company and Mr. Milton resigned as executive chairman.

“My approach is a friendly approach,” Mr. Adams said. “The majority of my audience is young people who are looking for insight into financial markets or looking for ways to be more strategic about the investments they’re making, whether it’s NFTs, or crypto, or a founder’s story, and company culture…. I don’t like to think of myself as an expert.”

’I don’t like to think of myself as an expert,’ Mr. Adams says of the relatable style he uses.

Photo: Michael Friberg for The Wall Street Journal

Rule 3: All bulls, no bears

Like most internet content, influencer videos thrive on popularity. And in the midst of a long-running bull market, what’s popular is success stories and hot tips only.

Many influencers report that when they hype an investment, they get the page views they crave. When the message is bearish, however, viewers turn away, or worse, attack the messenger with vicious trolling.

“For their entire adult life, the market has gone up. If you say otherwise, you just don’t get it,” said Scott Galloway, a marketing professor at NYU who has been trolled himself after posting skeptical videos about companies.

The real danger in the social-media finance world, he says, is that younger influencers tend to believe the market only goes up. Their followers reinforce this message.

“There is a surrender-to-the-narrative-or-else attitude online, and it’s really frightening, because if you say bitcoin is overvalued, or Tesla is overvalued or whatever popular SPAC is overvalued, these trolls in anonymous accounts come out of the woodwork and start attacking you,” Mr. Galloway said.

Critics of cryptocurrency investments are dismissed as spreaders of “FUD” (“fear, uncertainty, doubt”) in comment threads by trolls, a way of labeling an influencer as untrustworthy. Female online finance gurus have described sexual harassment in response to negative posts.

The GameStop frenzy put the spotlight on a growing group of investors who seek and share trading information on social media platforms like YouTube and TikTok. Three investors explain how these online communities are helping them chase the market. Photo illustration: Adam Falk/The Wall Street Journal The Wall Street Journal Interactive Edition

Mr. Paffrath, who is mounting a long-shot bid to become the next governor of California following a vote next month on whether to recall Gov. Gavin Newsom, says he is reminded of this dynamic weekly.

On a Wednesday afternoon in late May, he entered his home video studio wearing a necktie and a zip-up sweatshirt to record his daily live stream discussion about the stock market.

The day’s topic was AMC Entertainment Holdings Inc., the beleaguered movie-theater chain that saw its share price rocket this spring, swept along by individual investors, many of them young people who live most of their lives online, and the mania for so-called “meme stocks” including GameStop Inc. and Tesla Motors Inc.

AMC’s stock was in the midst of a volatile week, but that day, it happened to close up 10%. Mr. Paffrath cautioned against buying AMC shares—hedge funds appeared to be buying up short positions in the stock, and its share price could collapse at any moment, he said.

“Why you gotta be so negative lol” asked a viewer in the video’s live chat.

“I’m not being negative, I’m just being realistic,” responded Mr. Paffrath. “Look, I want AMC to go to the moon, but all I’m suggesting here is, if you’re hanging your hat on something you don’t understand…you just want to be careful.”

After the live stream ended, Mr. Paffrath started shedding thousands of subscribers, he said. Most videos with positive titles garner more than 200,000 views, he says, while videos that have negative takes on a company or an industry in the title rarely get more than 60,000 views.

“I have to be an astronaut and show them that the rocket ship is fun, but you can’t look at that your whole life,” Mr. Paffrath said.

“I’ll play the momentum game, but let’s be real: True wealth comes from long-run interest. The trouble is, on YouTube, none of my videos about that kind of thing get any views.”

Write to Robbie Whelan at robbie.whelan@wsj.com