His father Fe'ao captained Tonga in the 1990s when several uncles also represented the national side. His cousin Toby stars for Wales, while Vunipola and his brother Mako are standout players for England.

But that hasn't always made things easy, particularly during his early years.

Born in Sydney, raised in Wales and then England when the family relocated to the UK -- Fe'ao had signed to play for Pontypool -- the imposing number eight Billy describes his itinerant upbringing as "frantic," adding, "we were never settled anywhere until we came to England." Britain proved something of a culture shock.

"When we started setting up roots here, it was tough," says Vunipola. "I remember our first few months in Wales we all just slept in the living room because we were used to being around each other. We just had duvets all over the floor."

It's an endearing image and one that's difficult to picture today.

Vunipola (right) celebrates England's Six Nations victory over Scotland in 2016, when the team also won the grand slam.

Bulldozer

Vunipola tips the scales at 130 kg. He's a bulldozer with the ball in hand, but has honed his game in recent years to develop defensive nous and subtle handling skills.

Brother Mako, meanwhile, two years Billy's senior, is 9 kg lighter but equally adept in open play, eschewing the logic that prop forwards are supposed to focus solely on scrummaging.

Both will feature prominently in England's upcoming Six Nation campaign, which gets underway in Dublin against Ireland on February 2, with Vunipola having recently returned from a long injury layoff that saw him suffer three arm breaks.

"My name's on the sheet but I want to be back back," he says. "I want to be playing and I want to be playing well.

"I'm lucky that I only had to deal with broken arms, you know. I can still walk and I can still play rugby. It just took time out. It wasn't fun but I'm happy, I'm playing rugby and hopefully I can carry on.

"The Six Nations is probably the premier tournament outside the World Cup in terms of rugby, I would say, and in terms of viewership. The rugby that's been played, the unpredictability of the whole tournament, just as a fan it's exciting. As a player it's even more exciting."

After a disappointing fifth-place finish in last year's tournament -- for which Vunipola was absent through injury -- Eddie Jones' England side will be hoping for a much-improved Six Nations campaign with a World Cup on the horizon, and Vunipola's return should give confidence in that regard.

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Brotherly love

Since making his international debut in 2013, he's been a leading light in an England team that's endured unprecedented high and record lows, as well as helping his club side Saracens amass silverware on domestic and European fronts.

A rugby career, he says, always seemed inevitable.

"I remember growing up and my dad trying to push us in the direction of being doctors or lawyers or anything he thought was a good job," says Vunipola. "I'm not saying he was right or wrong but me and my brother always wanted to play rugby. We got our wish."

The seed was sown when the brothers started to wrestle with a rugby ball in their living room in Wales or played mini games with their cousins outdoors.

"Mako would be with Toby, who's now at Bath, and I would be with Toby's brother," recalls Vunipola. "Every single time we played it was always me and Mako that had a fight.

"I was always the youngest of the four of us. They'd both hate it if I actually got a successful tackle on them. He'd usually start [the fights] and finish them, but I knew that I was getting under his skin when I'd tackle him or score a try."

Billy and Mako celebrate with their family after winning the European Champions Cup in 2017.

That rivalry has mellowed now, with Vunipola even admitting that his brother is the superior player: "He's probably just a smaller, better, stronger version of me," he says.

Humility is something Vunipola has acquired over time. He says that his rise to the top of the game -- via a scholarship at prestigious English public school Harrow and the Wasps academy -- gave him undue self-confidence.

"If you keep hearing your name I guess subconsciously you just think you're the man and that's what I did. I thought I was more important than I actually was.

"I love seeing some of the young boys here at the club. You go in 50% and they go 100% and you get angry at them, but they do what's right. I've actually learned a lot from the younger guys coming in."

Photos: RWC 2019 venues New Zealand beat Australia 34-17 to win its second straight Rugby World Cup at Twickenham Stadium, London in October 2015. Four years on, the focus will shift to Japan, where 12 stadiums throughout the country will host the tournament from September 20 to November 2. Hide Caption 1 of 13 Photos: RWC 2019 venues What: International Stadium Yokohama

Capacity: 72,327

Where: Yokohama City, Kanagawa Prefecture

Matches: New Zealand vs South Africa; Ireland vs Scotland; England vs France; Japan vs Scotland; Semifinals 1 & 2; Final

Hide Caption 2 of 13 Photos: RWC 2019 venues What: Tokyo Stadium

Capacity: 49,970

Where: Tokyo Metropolitan

Matches: Japan vs Romania; France vs Argentina; Australia vs Wales; England v Argentina; New Zealand vs Namibia; Quarterfinals 2 & 4; Bronze final Hide Caption 3 of 13 Photos: RWC 2019 venues What: City of Toyota Stadium

Capacity: 45,000

Where: Toyota, Aichi Prefecture

Matches: Wales vs Georgia; South Africa vs Namibia; Japan vs Samoa; New Zealand vs Italy

Hide Caption 4 of 13 Photos: RWC 2019 venues What: Shizuoka Stadium Ecopa

Capacity: 50,889

Where: Shizuoka Prefecture

Matches: Japan vs Ireland; South Africa vs Italy; Scotland vs Romania; Australia vs Georgia Hide Caption 5 of 13 Photos: RWC 2019 venues What: Hanazono Rugby Stadium

Capacity: 32,228

Where: Higashiosaka City, Osaka Prefecture

Matches: Italy vs Namibia; Argentina vs Tonga; Georgia vs Fiji; USA vs Tonga Hide Caption 6 of 13 Photos: RWC 2019 venues What: Kumamoto Stadium

Capacity: 32,228

Where: Kumamoto City, Kumamoto Prefecture

Matches: France vs Tonga; Wales vs Uruguay Hide Caption 7 of 13 Photos: RWC 2019 venues What: Oita Stadium

Capacity: 40,000

Where: Oita Prefecture

Matches: New Zealand vs Canada; Australia vs Uruguay; Wales vs Fiji; Quarterfinals 1 & 3 Hide Caption 8 of 13 Photos: RWC 2019 venues What: Kumagaya Rugby Stadium

Capacity: 25,600

Where: Kumagaya City, Saitama Prefecture

Matches: Russia vs Samoa; Georgia vs Uruguay; Argentina vs USA Hide Caption 9 of 13 Photos: RWC 2019 venues What: Kamaishi Recovery Memorial Stadium

Capacity: 16,334

Where: Kamaishi City, Iwate Prefecture

Matches: Fiji vs Uruguay; Namibia vs Canada Hide Caption 10 of 13 Photos: RWC 2019 venues What: Sapporo Dome

Capacity: 41,410

Where: Sapporo City

Matches: Australia vs Fiji; England vs Tonga Hide Caption 11 of 13 Photos: RWC 2019 venues What: Kobe Misaki Stadium

Capacity: 30,132

Where: Kobe City

Matches: England vs USA; Scotland vs Samoa; Ireland vs Russia; South Africa vs Canada Hide Caption 12 of 13 Photos: RWC 2019 venues What: Fukuoka Hakatanomori Stadium

Capacity: 21,562

Where: Fukuoka City, Fukuoka Prefecture

Matches: Italy vs Canada; France vs USA; Ireland vs Samoa Hide Caption 13 of 13

While his initial focus is having an injury-free run of games through the Six Nations, Vunipola admits that his thoughts can't help but gravitate towards the World Cup in Japan later this year.

He was part of the England team that crashed out at the pool stages in 2015 -- the first host nation ever to do so -- and will be eager to right some wrongs in Japan.

"This is what you kind of prepare your whole life for," he says, "I guess I didn't understand that in 2015."

"We've got a Six Nations coming up, but, you know, it's something you can't be afraid to talk about. It's the World Cup, it's massively exciting. It's going to be a massive year for everyone."

Photos: The Rugby World Cup -- a history England, 2015 – New Zealand became the first side to retain the Rugby World Cup after defeating Australia 34-17 in the final at Twickenham Stadium. Hide Caption 1 of 8 Photos: The Rugby World Cup -- a history New Zealand, 2011 – The All Blacks ended a 24-year drought when they lifted the Web Ellis trophy on home soil in 2011. Here, Ma'a Nonu greets raucous crowds during the victory parade in Wellington. Hide Caption 2 of 8 Photos: The Rugby World Cup -- a history France, 2007 – South Africa claimed its second title in France in 2007, defeating England 6-15 at the Stade de France in Paris. Hide Caption 3 of 8 Photos: The Rugby World Cup -- a history Australia, 2003 – England became the first -- and to this day the only -- northern hemisphere side to win the World Cup in 2003 by defeating Australia 20-17 in Sydney. Fly-half Jonny Wilkinson struck the winning drop goal in extra-time. Hide Caption 4 of 8 Photos: The Rugby World Cup -- a history Wales, 1999 – Australia's second World Cup victory in 1999 came eight years after its first. A 35-12 victory over France saw John Eales' side lift the trophy in Cardiff. Hide Caption 5 of 8 Photos: The Rugby World Cup -- a history South Africa, 1995 – It's one of rugby's most iconic images -- South African President Nelson Mandela presenting the World Cup to Springbok captain Francois Pienaar in 1995. The victory helped unite the nation shortly after the end of apartheid. Hide Caption 6 of 8 Photos: The Rugby World Cup -- a history England, 1991 – Australia first tasted World Cup victory in 1991 after narrowly defeating England 12-6 in the final. Hide Caption 7 of 8 Photos: The Rugby World Cup -- a history New Zealand, 1987 – The All Blacks won the inaugural Rugby World Cup on home soil in 1987. Captain David Kirk kisses the trophy following victory over France in Auckland. Hide Caption 8 of 8

Japan 2019

England's opening match of the tournament -- against Tonga -- will divide the Vunipola household. Fe'ao won 32 caps for the South Pacific island and today works for the Tonga Rugby Union.

"I guess we're his sons and he'll want us to do well but you know deep down inside he wants Tonga to secretly get one over England," says Vunipola.

"I know my mom will 100% be behind us and the rest of my family but my Dad will be a bit torn."

Born in Australia, raised in the UK and crowned world champion in Japan? Vunipola's story would enter another thrilling chapter if England were to defy the odds and lift the World Cup.

"The thing is I'm one of millions of people that think about winning in Japan and I guess I'm one of the millions that aren't scared to talk about it," he says.

"All these cliché things that you can sit here and say in an interview is correct for right now. It's a dream."