Her father, Choi Tae-min, was well known in South Korea as the head of a cult-like religious group until his death in 1994.
He referred to himself as the "Future Buddha" and established a sect called the Eternal Life Church. Ms Park has publicly described herself as Catholic.
The elder Choi reportedly enjoyed considerable influence over Ms Park as a young politician, prompting diplomats to refer to him in private as "Korea's Rasputin."
A newly released Wikileaks cable from the US embassy in Seoul described him as having "complete control over the body and soul of the president in her formative years" in 2007.
Both women are understood to have met decades ago and formed a close relationship when Ms Choi's father allegedly helped the future president contact her late mother in the afterlife.
Since then, according to South Korean media reports, the pair have been inseparable - though Ms Choi has never held an official position in the country's government, nor did she have security clearance.
The disclosure has sparked a tidal wave of anger in South Korea and has led to the country's once-celebrated leader facing potentially the end of her presidency and even impeachment.
Last Friday she ordered her entire cabinet to resign at the behest of senior party officials after her approval ratings plunged to an historic low of just 17 per cent.
And on Saturday, prosecutors confiscated computers and documents from the homes of a top presidential adviser and two other aides as well as a deputy culture minister.
They also searched offices in the presidential Blue House complex, according to Yonhap News Agency.
A table computer seized during the raid is said to contain documents which show Ms Choi edited a landmark speech the president gave in Germany in 2014 which outlined her unification plans with the North.
Ms Park denies she ever had an improper relationship with Ms Choi.
Ms Choi, who is currently in Germany, says she is too ill to return to Seoul for questioning over the affair.
"I am suffering from a nervous breakdown and I have been diagnosed with heart issues," she told Segye Ilbo, a South Korean newspaper.
"If I recover, I will ask for forgiveness, and will accept punishment if I did anything wrong," she added.
The newspaper also reported that Ms Choi had privileged access to sensitive South Korean military intelligence, citing sources close to the president.
Her lawyers say she is co-operating with the investigation and will return to the Korean peninsula when she is able to.
Prosecutors have taken in two of Ms Choi's close aides for questioning, including one who told reporters that she had been behaving as Ms Park's "de facto regent".
Shin Yool, a professor at Myongji University, told the Washington Post that the country is facing its "biggest crisis" since it was founded 70 years ago.
"The president has lost her ability to function as leader," she said.
Kim Jong-un, the North Korean leader, is said to be delighted by the scandal.