Sara Mardini is in a chipper mood. Freedom, the Syrian readily acknowledges, is a precious thing. For each of the 107 days she spent behind bars in Athens’ high-security Korydallos jail, she clung on to the hope that a trial would prove her innocence.

“There were times when I’d say, ‘I can’t do this any more’,” the refugee-turned rescue worker said shortly after her release this month. “My heart was very heavy.”

Mardini, a former competitive swimmer, made international headlines along with her sister Yusra after the pair jumped into the sea and towed a sinking dinghy for three hours through the turbulent waters of the Aegean, saving the lives of 18 fellow passengers onboard. She was arrested in August along with three other charity workers on charges of people-smuggling, spying, violation of state secrecy laws and money laundering.

By the time a guard broke the news that she was to be freed, Mardini was so mentally and physically exhausted she was unable to leave her bed.

“In jail you are a ball of emotion,” she says. “A phone call from someone outside could blow my brain. And then suddenly on a day when I am feeling really bad, when my body seems to have stopped responding and I can’t even lift my little finger, I am told, ‘You will be freed [on bail].’ And all I feel is shock. Shock at everything really.”

At 23, Mardini is now the face of humanitarianism under fire. Coming three years after her own dramatic landing on the island of Lesbos, her arrest, the charges against her and her long incarceration pending trial caused international outrage. She is the most high-profile victim of what humanitarian operations have said is the criminalisation of volunteers for charities and aid groups detained across Europe. On 5 December she was finally freed after her lawyers posted a €5,000 (£4,450) bail.

While she was locked up, Mardini says she meditated, exercised, attended art classes and spent a lot of time sleeping, in the hope of blocking out what she remains acutely aware of: under Greek law the crimes she was charged with carry penalties of up to 25 years in prison.

“I want this to go to trial, I want my freedom back,” she says. Mardini, who has settled in Germany, returned to Lesbos to work as a search and rescue volunteer with the Emergency Response Centre International, a Greek NGO. “I think we will know all the truth when it gets to trial and I won’t feel free until I am over with it, until I hear I am innocent.”

The Mardini sisters were among a group of 20 Syrians making the treacherous sea crossing in a small dinghy from Turkey to Greece in August 2015, when their dinghy began taking in water, and the siblings, both competitive swimmers, jumped overboard to haul and push the boat, reaching Lesbos more than three hours later.

Yusra, who has since become a UNHCR goodwill ambassador, went on to participate as a refugee team member in the 2016 Rio Olympic games and the sisters are the subject of an upcoming film.

Yusra Mardini competes in a Women’s 100m Butterfly race at the Rio 2016 Olympic Games, as part of the Refugee Team. Photograph: Christophe Simon/AFP/Getty Images

Yet the drama swirling around Sara has dominated headlines. To her friends, Mardini is selflessly dedicated to doing good. “I am a card with two faces,” she says, rolling a cigarette between painted fingernails. “I have experienced being inside and outside the boat, I am a rescuer and a refugee. Every single boat I have ever helped, I have felt in my bones.”

But for others, Lesbos is at the centre of a murky underworld of people-smuggling. Local officials and police claim that, of all the philanthropic organisations working on the island, the Greek ERCI has aroused most suspicion in tracking migrant arrivals and – according to one police report – bringing people illegally ashore.

The group, which has been credited with rescuing hundreds of people at sea, has now ceased operations. Organisations that rescue migrants have come under fire with some claiming they are a “pull factor” for migrants in regions at the frontline of the migration crisis.

Sara Mardini shows little anger over what has happened to her. Her months behind bars have made her want to help improve conditions in Greece’s toughest prison.

“I will keep in touch with people there – the guards, the prison manager. They were all very lovely. You could see they could feel our pain,” she says. “I am not usually an emotional person. I have learned how to deal with hard emotions … the only thing I do know, however, is that none of us [in ERCI] has done anything illegal. We have very strict rules that we have always followed.”

She will spend the next few months doing what she had intended to do before her arrest at Lesbos airport in August: studying at the liberal arts school in Berlin where she has been accepted on scholarship.

But the fresh-faced student also has a message.

Sara Mardini has settled in Germany, but returned to Greece to volunteer with a search and rescue team. Photograph: Helena Smith

“People should never be afraid of what they don’t know,” she says. “I worry about conflicts not only in Syria, in Africa, Afghanistan, in so many places. Refugees will keep on coming and yet they are not only refugees, they are doctors, engineers, teachers, they can help in Europe too.”

Is she upset her that, in the wake of her own ordeal, volunteers reportedly started leaving Lesbos?

“Volunteers won’t have it easy,” she says. “They will feel discouraged, but they should know that when they make someone happy, when they make someone smile, it really does mean the world to a person who has lost everything.”