Microsoft co-founder Paul Allen. Photo / AP
Paul Allen was one of the richest men on Earth — but before his death earlier this week, most people had barely heard of him.
Allen co-founded Microsoft with Bill Gates in 1975, but while Gates became a household name, Allen managed to keep a far lower profile.
After being diagnosed with Hodgkin's lymphoma in 1982, Allen stepped away from the company, allegedly splitting it 60-40 in Gates' favour.
The following year, Gates reportedly attempted to buy his former partner out, offering him US$5 ($7.65) per share.
Allen refused — and went on to be dubbed an "accidental zillionaire" by the press when the groundbreaking company went public after he left the business.
In fact, when he died from cancer at the age of 65 this week, Allen left behind a staggering US$20.3 billion fortune.
But despite being named by Forbes as the 44th richest person on the planet — and with a penchant for super yachts, mansions and lavish parties — the fiercely private magnate cleverly managed to avoid too much media attention over the years.
However, he quietly enjoyed a life of almost unbelievable luxury.
In 2003, Allen launched his 126m, US$200 million yacht, Octopus, which boasted a 60-person crew, seven boats and its own submarine.
According to Business Insider, the yacht costs US$384,000 per week to run — and was just one of three in the billionaire's private fleet.
It has its own recording studio, which has been used by the likes of Mick Jagger and the Beastie Boys, while other big names including Usher, U2 and Johnny Cash have all reportedly performed on-board the vessel, testament to Allen's lifelong love of rock music.
Allen, who never married and had no children, also enjoyed hosting extravagant parties on board, attended by celebrities including model Jerry Hall, actor Carrie Fisher and director James Cameron, who were guests at his 45th birthday cruise to Alaska.
In 2008, he hosted a multimillion-dollar New Year's Eve bash in St Bart's, with Steven Spielberg, Denzel Washington, Jon Bon Jovi and Billy Joel among the glittering guest list.
But despite his love of excess, Allen was also notoriously private, reportedly making guests sign a nondisclosure agreement before attending his parties in a bid to stop information leaking to the media.
Allen was also an art lover, collecting priceless works by masters including Monet, Renoir, and Rodin.
And he amassed an impressive real estate portfolio, including a mansion on the French Riviera, a cinema in Seattle, and a New York penthouse apartment which he paid US$25m for, Business Insider reported. He even bought Allan Island off the Washington coast in 1992, but sold it in 2013 when its seclusion made it too difficult to build his dream home there.
And naturally, he also had a collection of sports cars and private jets, which he stored at his own hangar at an airport in Seattle, according to the Daily Mail.
He forked out US$200m to buy his own football team, the Seahawks, as well as US$70m for the Portland Trail Blazers basketball team.
And like fellow billionaires Jeff Bezos and Elon Musk, Allen was also fascinated by space, investing hundreds of millions into space exploration and technology development.
But Allen didn't just splurge on his own entertainment.
During his lifetime, he donated more than US$2b to a slew of charities and organisations dedicated to science, the arts, conservation, community services, technology and education.
He also established the Paul G. Allen Family Foundation to oversee his philanthropy projects, and signed up to The Giving Pledge, a campaign which urges the elite to give most of their fortunes to worthy causes.
But despite his incredible wealth, business success, generosity and flashy lifestyle, Allen, who is survived by his sister Jody, still managed to remain largely anonymous during his life — just as he planned.
"If somebody runs into me at a party, typically they have no idea if I'm just one of the guitar players in the band or if I'm the host," he famously once said.