Choi Soon-Sil, a long-time friend of President Park Geun-Hye is accused of taking advantage of her personal connections with the Head of State to interfere in government affairs.
South Korean prosecutors on Monday questioned the woman at the center of a political scandal that has shattered public confidence in President Park Geun-Hye, with allegations of fraud and meddling in state affairs.
Suggestions that Choi Soon-Sil, a close friend with Park, vetted presidential speeches and was given access to classified documents has exposed the Head of State to public anger and ridicule and, with just over a year left in office, pushed her approval ratings off a cliff.
There has also been mass street protests in Seoul and other cities to demand Park's resignation. Meanwhile, Choi, who has denied any criminal wrongdoing has handed herself over to prosecutors in Seoul a day after flying back from Germany, begging for forgiveness.
"Please forgive me. I have committed a deadly sin," Choi was quoted as saying by local media as she arrived in Seoul District Prosecutor's office on Monday.
She was dressed head to toe in black and covering her face with a hat and scarf.
Park and Choi have been close friends for 40 years. The precise nature of that friendship lies at the heart of the current scandal which has triggered a media frenzy in South Korea, with lurid reports of religious cults and shamanistic rituals.
The media has portrayed the 60-year-old Choi as a Rasputin-like figure, who wielded an unhealthy influence over Park and interfered in government policy despite holding no official post and having no security clearance.
A task force, led by the head of the Seoul prosecutors' office, has been set up to investigate the leak of presidential documents and whether Choi meddled in state affairs.
Choi has also been accused of using her relationship with the president to force corporate donations to two non-profit foundations and then siphoning off funds for her personal use.
"We hope that the various allegations can be thoroughly verified," presidential spokesman Jung Youn-Kuk told reporters.
Choi is the daughter of a late shadowy religious leader and one-time Park mentor called Choi Tae-Min, who set up his own cult-like group known as the Church of Eternal Life.
Choi Tae-Min befriended a traumatised Park after the 1974 assassination of her mother, whom he said had appeared to him in a dream, asking him to help her daughter.
He became a long-time mentor to Park, who subsequently formed a close bond with Choi Soon-Sil that endured after Choi Tae-Min's death in 1994.
Choi Soon-Sil's ex-husband served as a top aide to Park until her presidential election victory in 2012.
A public apology by Park, in which she acknowledged seeking limited advice from Choi, did little to assuage public outrage and she has struggled to draw a political line under the crisis.