Sarah Mardini, 23, was released on bail in early December, and has since returned to Berlin.
“I'm in shock about the situation and how fast everything happened,” Mardini told AFP.
“They arrested me because I've been accused of smuggling migrants… and for being part of a criminal organization,” she said.
However, Mardini said she had only been doing “what we can to save the refugees”.
Mardini and her sister Yusra made headlines in 2015, when they used their swimming skills to pull to safety their water-logged boat with another 18 people onboard on its journey from Turkey to Lesbos.
The following year Yusra, now a UNHCR goodwill ambassador, was on the refugee team at the Rio 2016 Olympics and Mardini went on to win a scholarship studying economics and social sciences at Bard College in Berlin.
Mardini, who worked as a volunteer for the NGO ERCI on the Greek island of Lesbos, was arrested and placed in provisional detention at the end of August.
She has been charged with participation in a network that allegedly facilitates irregular immigration.
Her lawyer Zacharias Kesses meanwhile accused Greek authorities of trying to “criminalize” the NGO. “What they want is to send a message to the NGOs that they should not only cooperate with the authorities but that they should also work for them,” he added.
Mardini and her close friend Sean Binder, who was also imprisoned in Athens, cannot understand the accusations, the Tagesspiegel reported. We have left our friends and families, good educational opportunities and our comfortable home to work there 24 hours a day,” said Mardini. “That's not fair.”
Binder is also stunned. He said: “There is nothing wrong with helping people, saving them from the water or showing children how they can be children again.”
The news of her release brought pure joy to Mardini. “I couldn't believe I was finally able to move alone again outside,” she said, the Tagesspiegel reported.
Her future remains uncertain but for now, Mardini wants to return to her school. And she is determined to continue supporting refugees.
The months before the arrest were the best of her life. “I want to encourage people to carry on, they must not be afraid of volunteering.”
Mardini's imprisonment had sparked outrage among human rights activists.
Lesbos has been a key gateway into the European Union since the start of the bloc's migration crisis in 2015.
At the height of the influx, some 5,000 migrants and refugees, mostly from war-torn Syria, landed on the island's beaches on a daily basis.