Ukrainian army pilot Nadezhda (Nadiya) Savchenko holds a sign inside a defendants' cage as she attends a hearing at the Basmanny district court in Moscow on Feb. 10. (Maxim Zmeyev/Reuters)

After 15 hours of grueling, nonstop talks, world leaders gathered in Minsk, Belarus, announced a deal for a cease-fire in Ukraine on Thursday. “We have agreed on many things,” Russian President Vladimir Putin told reporters.

The exact details of the deal will no doubt be ironed out, debated and argued about in the coming days, but right now it appears that the Ukrainians are hoping for one clear concession: Russia's release of Nadiya Savchenko.

Savchenko, a 33-year-old former military navigator, has been well-known in Ukraine for years. She was one of the first Ukrainian women to train as an air force navigator and was featured in a documentary made by the Ukrainian military in 2011. When Ukraine descended into chaos in early 2014, she joined the Aidar Battalion, a volunteer group, to fight pro-Russian separatists in the country's east.

There are disputed stories about exactly what happened next -- she claims she was captured by separatists and handed over to Russian forces -- but Russia says she tried to enter the country as a refugee. Either way, she ended up being held by Russian forces, and in July, she was charged in connection with the deaths of two Russian state television journalists in June. She faces 20 years in prison.

Now, after months in detention in Moscow's notorious Matrosskaya Tishina prison, Savchenko has become an emotive focal point for both Ukrainian and international criticism of Russia. She's been called an illegal "prisoner of war" by her lawyer (which Russia denies), and both the United States and the Parliamentary Assembly of the Council of Europe have called for her release. She's accused of supplying coordinates for a mortar strike that killed the two journalists, but her supporters point to mobile phone records that they say prove she was captured before the strike occurred and that she should never have been held by Russia anyway.

Throughout it all, she's cut a defiant figure. Shortly after she was captured, a video surfaced on YouTube that showed Savchenko being interrogated by a number of men off camera. Despite being handcuffed to a pipe and surrounded by armed men, she refused to play along with her captors: When asked who is fighting pro-Russian separatists, she replies: "I think all of Ukraine."

When she's appeared in court, she's refused to speak Russian, worn T-shirts showing the Ukrainian trident, and held up signs reading: "I was born Ukrainian, and I die Ukrainian." For the past two months, she's been on hunger strike, drinking just two liters of warm water each day, and is drip-fed glucose, her lawyers and family told Reuters. Her supporters fear that she might die.

Online, pro-Ukrainian groups treat her like a national hero. Her plight has spawned a number of popular hashtags, including #SaveOurGirl and #FreeSavchenko, where fans share photos and drawings and call for her release.

“She’s our Joan of Arc,” Valeriy Ryabykh, a military expert with the Defense Express magazine, told The Washington Post last year.

With this widespread popular support, the Ukrainian navigator became a political cause. Savchenko became a deputy with the opposition in Ukraine's parliament in November, and Ukraine's government has made her a national cause.

“Nadiya Savchenko is a symbol of struggle for Ukraine," Ukrainian President Petro Poroshenko said in October. "While being in captivity, she demonstrated true, strong, martial Ukrainian spirit of a serviceman who doesn’t betray [the] Motherland.”

Now Poroshenko says Savchenko should be released as part of a new deal under which both sides would release prisoners. "I raised the issue of the release of Nadiya Savchenko and I was informed that it should be done soon after the medical examination and the preliminary findings of the investigation are finalized," Poroshenko told reporters Thursday, adding that he had the support of German Chancellor Angela Merkel and French President Francois Hollande as well.

Whether Russia would actually acquiesce and release Savchenko is not confirmed, however. In the past, the Kremlin has said it was a matter for the courts and refused to influence them. "An automatic release mechanism for someone accused of complicity in the murder ... does not exist," Alexei Pushkov, a prominent lawmaker and head of Russia’s parliamentary committee on international affairs, told reporters Thursday.