He's best known for the financial nous that made him the world's richest man - until he started giving it away.

But a more complex side to Warren Buffett has been exposed in a HBO documentary, Becoming Warren Buffett, about the Berkshire Hathaway chairman, including the unorthodox relationship between the billionaire investor and his late wife, Susan, news.com.au reported.

Although the pair had a bond that endured over decades, Buffett and his wife spent more than half their 52-year marriage living apart, after Susan moved to San Francisco to pursue a singing career in 1977.

Susie and Warren Buffett raised three children together before deciding to live apart. Photo / Supplied
Susie and Warren Buffett raised three children together before deciding to live apart. Photo / Supplied

What made it truly unique was the fact that, instead of separating, Susan sent a female friend to live with Buffett - and she became his live-in companion.

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Christmas cards sent to friends of the Buffett family were signed by the trio, and Astrid Menks married Buffett after Susan's death.

"Astrid has lived with me for a long time, she's done wonders for me," Buffett said in the documentary, released this year.

"It worked well, but I don't think it will work for lots of other people necessarily."

Buffett had three children with Susan, whom he wed in 1952 when she was just 19, and he was 21.

The pair had met when Susan was about to start college, and Buffett's sister was her roommate.

Her first impression was not a flattering one.

"I thought, 'who is this sarcastic jerk?'" she said in archival footage.

"My dad said 'you're not going to have discussions with him like you will with normal people'."

The prediction proved true and, as Buffett's three adult children recalled, he was often distracted at home, holing up in his room to pore over company reports - although he always came home for dinner.

Buffett was "not like the rest of us", son Peter said. "I don't think my dad ever took anybody for granted, but you are a little bit blind sometimes to what other people might be doing behind the scenes, and my dad's gotten a little bit of a pass."

Susan was a smart, independent woman who was heavily involved in the civil rights movement and has been credited with shifting Buffett's allegiance away from the Republican party in which they were raised.

She was described as a "balancing force" who "softened" Buffett, and whom he did not know how to survive without - although he often wouldn't have noticed if she was home or not.

"Physical proximity to Warren doesn't always mean that he's there with you," she said in an interview before her death.

"He's so cerebral, you see? And that's why I learned to have my own life. We were two parallel lines, but very connected when he was open to connecting."

Buffett described Susan as "a better person than I was" when the pair wed, and "much more mature".

"Susie really put me together, she believed in me," he said.

"And I would not only have not turned out to be the person I turned out to be, but I actually wouldn't have been as successful in business without that. She made me more of a whole person. Over the years, I've got a better understanding of human nature."

Although his children recalled him being distraught and "unable to function" after Susan left, he supported her choice to move away and start a new life.

Susan Buffett, left, with Astrid Menks and Buffett's daughter Susie. Photo / Supplied
Susan Buffett, left, with Astrid Menks and Buffett's daughter Susie. Photo / Supplied

And while his relationship with Astrid deepened, he and his wife remained close, holidaying together and appearing side by side at Berkshire Hathaway meetings, where they would entertain shareholders with a song and dance.

"Susie and I loved each other, we admired each other and we were totally in sync with what the other was doing. But we were two different individuals," Buffett said.

Daughter Susie said that most people would not understand her parents' unusual relationship, but it worked.

"As strange as it may seem to people, I just think 'who cares, if it's working between the people who are directly involved - who cares what anyone thinks?'"

And, she said, "my mother and Astrid were very close, they really, really loved each other. And I think that my mother was glad that she was there."

When Susan was undergoing surgery and radiation treatment for oral cancer in 2003, Buffett flew to see her every weekend - and cut his daily calorie intake to match her meagre liquid diet.

She died the following year, aged 72, after suffering a stroke.

Having to wake Buffett up and deliver the news was incredibly difficult, daughter Susie recalled.

"He was sitting there all night holding her hand," she said.

"I was so proud of him because, when it came down to it, he knew what he was supposed to do and he did it - which was nothing."

Buffett, who throughout his time as the world's richest man was criticised for his lack of philanthropic activity, explained Susan's influence on his decision to start giving away vast swathes of his wealth in 2009.

Last year, he gave US$2.86 billion ($3.93b) worth of Berkshire Hathaway stock to a handful of charities, on top of $2.84b he gave in 2015 and $2.8b in 2014, and has promised to eventually give away 99 per cent of his wealth.

"My wife Susie and I had planned that whatever I made would go back to society, and originally I thought she would outlive me and that she'd made a big decision on it," Buffett said.

"But, since her death, I had to rethink the best way to get the money into society and to have it used in the most effective way."

- news.com.au