May 14 (Reuters) - Retail investor darlings GameStop and AMC soared on Tuesday, as posts from "Roaring Kitty" Keith Gill raised chatter about the return of the central figure behind the 2021 meme stock frenzy.

Shares of both video game retailer Gamestop and those of the world's largest theater chain AMC more than doubled during the trading session. Gamestop shares rose to the highest since June 2021, last up 44% at $43.61. AMC shares notched the biggest gain since January 2021, last up 33% at $6.83.

The rally began on Monday after Gill shared a meme and more than 10 clips from movies, including "X-Men Origins: Wolverine," "The Avengers" and the 1993 Western "Tombstone."

While the posts did not mention company names, GameStop and AMC were the most-traded stocks by retail investors on Monday and Tuesday as of 10:30 a.m. ET, J.P.Morgan data showed.

That was mainly because Gill, who is returning to social media platform X after nearly three years, is credited with sparking the so-called Reddit rally in January 2021 with bullish calls on GameStop.

"The fact that Roaring Kitty is back should be totally meaningless to the stock market (but) the fact that it isn't is fascinating," said Matthew Tuttle, CEO of Tuttle Capital Management.

GameStop's shares have nearly tripled in value since Friday's close, its market capitalization jumping to $16 billion.

On Tuesday, short sellers have lost $1.6 billion on paper, taking combined losses since Monday to $2.4 billion, analytics firm Ortex Technologies said.

"Today's losses will put a lot of short sellers on tilt and squeeze them out of their positions with their buy-to-covers pushing GME's stock price even higher," said Ihor Dusaniwsky, managing director of predictive analytics at S3 Partners.

Meanwhile, AMC completed a $250 million "at-the-market" share sale program on Monday as its stock surged 78% to $5.19, more than double its record low hit in mid-April.

The frenzy spread to micro-cap shares such as Headphones maker Koss which jumped 29%. U.S.-listed shares of BlackBerry rose 10%, and food storage container company Tupperware gained 7%.

Reddit added 7.4%. Retail investors used the social media firm in 2021 to coordinate and target highly shorted stocks as Main Street battled Wall Street.

Retail investor-focused brokerage Robinhood, which made zero-commission trades mainstream, gained 6%.

"This is ridiculous but all a part of free markets," said Andrew Left at Citron Research, a former GameStop short seller.


Unlike 2021, speculative trades are not likely to last long in the current economic condition, some analysts, noting interest rates are much higher than in the era of cheap money.

"This meme rally maybe rhymes with 2021 but is unlikely a repeat," said Ben Laidler, global markets strategist at digital brokerage eToro.

About 600,000 GameStop options contracts changed hands on Monday, far below the volumes of one million to two million contracts in early 2021, said Brent Kochuba, founder of options analytic service SpotGamma.

However, the implied volatility of GameStop options, a measure of investor expectations for price swings in the shares, jumped in the previous session to levels that rivaled 2021.

"This suggests that the reaction of market makers (increasing options prices) to a potential repeat of GameStop mania was very fast," he said.

Institutional investors are now better equipped to handle the situation, market participants said, after the surprise collective force of individual investors cost hedge funds billions in losses three years ago.

In the past five sessions, the average retail traders' share of total GameStop turnover was around 7% and about 10% for AMC, said Marco Iachini, senior vice president at Vanda Research.

He said that indicates large numbers of institutional investors were participating along with retail investors.

(Reporting by Medha Singh, additional reporting by Pranav Kashyap in Bengaluru and Chibuike Oguh in New York; Editing by Krishna Chandra Eluri, Arun Koyyur and David Gregorio)