Her fans call her Heidi. She is 36 years old. She talks a mile a minute. She has a tattoo of the Polish-German revolutionary Rosa Luxemburg on her left arm and a million followers across TikTok and Instagram. She was relatively unknown in German politics until January, but as of Sunday, she’s a political force.

Heidi Reichinnek is the woman who led the surprise story of Germany’s parliamentary elections on Sunday: an almost overnight resurgence of Die Linke, which translates as “The Left.”

A month ago, Die Linke looked likely to miss the 5 percent voting cutoff needed for parties to earn seats in Germany’s Parliament, the Bundestag. On Sunday, it won nearly 9 percent of the vote and 64 seats in the Bundestag. “It was one of only five parties to win multiple seats in the new Parliament, joining the Christian Democrats, the Social Democrats, the hard-right Alternative for Germany and the Green Party.

It was a remarkable comeback, powered by young voters, high prices, a backlash against conservative politicians, and a social-media-forward message that mixed celebration and defiance.

At a time when German politicians are moving to the right on issues like immigration, and when the Alternative for Germany, or AfD, doubled its vote share from four years ago, Ms. Reichinnek, the party’s co-leader in the Bundestag, and Die Linke succeeded by channeling outrage from liberal, young voters.

They pitched themselves as an aggressive check on a more conservative government, which will almost certainly be led by Friedrich Merz, a businessman who has led the Christian Democrats to take a harsher line on border security and migrants.