School drop-out, video-game obsessive, multi-millionaire.

You may not have heard of Olajide Olatunji - better known by his alias, KSIOlajidebt, or KSI for short - but there’s a good chance your children, nieces or nephews will be very familiar with him.

The 21-year-old is one of the UK’s most popular digital media superstars, boasting more than 9,000,000 subscribers to his two YouTube channels (to put that into context, Tom Cruise has just 4,000), and a portfolio of brand endorsements to rival David Beckham.

His videos - bombastic, bawdy, borderline - have been watched more than a billion times.

The concept behind his spiralling success (he is already very wealthy, but more on that later) may baffle those whose teenage years are behind them, but it has been hugely effective.

Olatunji films himself playing video games - usually his beloved Fifa - and then uploads the footage to YouTube.

This is the brave new world of social media: a meritocracy which rewards mass popularity very handsomely indeed.

He makes up to 40 videos a month. Each is watched around two million times. The business model is simple: the more clicks his videos receive, the more cash he earns (through YouTube partnership programmes, which split the revenue from pre-roll adverts).

Not that he concerns himself too much with figures and projections: “I have an agent for that, I’m not business-minded.”

While his subject scope has widened, it is Fifa, the Electronic Arts football game for which he is best known.

“Fifa is my baby. I have to pinch myself sometimes… I am living a lifestyle most guys my age would love to lead, playing computer games and earning good money.

“I would say in a year I spend maybe half of that playing Fifa. I love it that much. I think I must have spent 200 hours on the new game already.

“It’s pretty cool I’m known for Fifa," he adds. "If you think of KSI, people instantly think ‘ah, that Fifa YouTuber’. I didn’t think I would be up there as the ambassador of Fifa on YouTube, but I’ll take it.”

It’s been a rapid ascent since his first foray into the world of YouTube in 2009, with clips filmed in his bedroom at his parents’ home in Watford.

But his rise hasn't been without controversy. Olatunji has been widely criticised for his so-called 'rape face' videos, and in 2012 he was banned from attending future Eurogamer Expo events as a result of inappropriate behaviour towards women during the show.

His manager later issued an apology, arguing that the Eurogamer controversy could partly be attributed to youth (he was 18 at the time) and inexperience. He added: "KSI is a character of Olatunji, a young man with great humility and politeness and far from the character portrayed in the Eurogamer video. Sexism is not something he condones or wants to be associated with."

Some in the gaming community have argued that Olatunji still hasn't learnt his lesson, and has been permanently tarnished by the sexism allegations, but the controversy doesn't appear to have harmed his bankability, as the queue of firms keen to sign him up demonstrates.

In person, just as his management claims, Olatunji - or JJ as he is known to friends - is nothing like his controversial digital alter-ego. He is refreshingly down to earth, polite and younger looking than he appears on screen.

There is an innocence about his success, too. The fame and fortune he has attracted (he now lives in a suburban pile in Kent with two highly desirable cars in the driveway) was never part of the plan. He was just having fun.

“I didn’t really want to become huge, I never saw myself as being a big celebrity. I just wanted to do videos because I enjoyed doing it and I saw people were making a bit of money.

"I thought I wouldn’t mind doing this as a job, getting just enough to survive and doing what I love. But now, it’s become so big.”

With fame comes responsibility, even in the free-for-all world of YouTube.

“When I started out, I was able to do whatever I wanted. I would play with more ideas and push the boundaries a lot. But nowadays I’m not able to do that so much.

“The amount of people that watch me," he says, "they are influenced by a lot of the things that I do. I wouldn’t want to annoy people's parents."

He seems to be aware that any further controversy could harm the KSI "brand".

"KSI, if you want to talk business, is a brand, and I have to do stuff to protect that brand now so it doesn’t become tarnished and I am not seen as a hated person, or in a bad light."

He still seems shocked at how playing football video games has given him the opportunity to meet real-life football stars, who are keen to associate with KSI.

There was an invitation to play Fifa with Rio Ferdinand at the former Manchester United defender’s restaurant to help develop his #5 YouTube channel and brand ("It was amazing that playing Fifa had got me into that position").

Arsenal’s Kieran Gibbs, Wojciech Szczęsny and Alex Oxlade-Chamberlain (“the Ox was probably the best I came up against”) are among other footballers to have played Fifa with KSI.

Beginnings

Being his own boss, Olatunji sets his own routine. We meet at 3pm because his manager warns me he rarely rises before noon (and rarely goes to bed before 5am).

His house is like a teenager's fantasy (giant TVs, empty cider bottles, full-size ping-pong table) and a mother's nightmare (all of the above plus a bedroom littered with discarded clothes, a smashed up lap-top, the legs from a skeleton, an inflatable penis and much, much more).

He speaks passionately as he explains where it all began: “I was big on gaming and I really wanted to explore the idea of uploading videos to YouTube, especially gaming videos.

"So I tried it myself and I didn’t do very well, but it was a start.

"I remember my first video - it had zero views, it was so depressing. People were getting thousands of views and I was sitting with my zero.

“I pushed it out to my family and friends, and on to forums, and eventually it started to get a bit more traction.

“After a year, I got to 7,000 subscribers, so that was pretty crazy. After another year, it went up to around 20,000.

"Back then, to grow was extremely hard. It took a lot of time and eventually I started posting more and more and more, more Fifa videos, more real life videos."

He says one of the first "big videos" was the History of my Name but admits it was also one of the "most cringeworthy". "I hate watching it now, he says, "but from that video people really started to relate to me.”

Given that the rise of the YouTube megastar is a difficult concept to grasp for anyone much older than Olatunji, it’s hardly surprising that he had difficulty persuading his parents that quitting school for a career playing computer games was a good idea.

“I wasn’t really into school that much. I was in this building having to cram knowledge I didn’t really care for. But on YouTube I was able to create what I wanted and post it for people to watch.

“I told my teacher that I wasn’t enjoying school and that I was enjoying YouTube more, and that I was getting paid good amounts.

<noframes>Interactive chart: Most Viewed UK YouTube Channels, August 2014</noframes>

"I asked the teacher, ‘should I leave?’ He asked, ‘how much are you making from YouTube?’ and I said around £1,500 a month. He told me he was getting less than that."

But parents were furious when he told them he was quitting school.

“They said this is the dumbest thing you have ever done, you are going to throw your life away, why would you just want to play games, you can’t make money from games, it's stupid.

“Now they have completely changed. They're doing videos with me and my brother all the time and they completely get it.

“I was able to pay for my parents’ house which was really, really cool, it was the best thing I could ever do for them. They have looked after me so it was awesome for me to be able to look after them.”

Wealth

You can hardly blame a 21-year old from a modest background for splashing the cash when it suddenly lands in his lap. Olatunji says he spends his money on things like trainers (there are limited-edition pairs strewn everywhere). His latest sports car was one of his more extravagent purchases: "I bought the car last year... I hardly ever splash so I thought I might as well get something really good."

But despite his protestations that he has no business brain, he seems surprisingly mature when it comes to planning for the future. He also recognises that the crest of a wave he is currently cyber-surfing won't roll on forever.

"I guess I spend my money quite wisely. I bought my old house off my parents, and now I am renting it out.

"I am trying to make sure that I don’t spend on ridiculous things, so that after all this YouTube thing goes I’m not left there, like, ‘uh oh, I have nothing’."

For the meantime though, his success continues. KSI's YouTube subscriptions are growing at an incredible rate, the money-spinning deals are coming thick and fast, and he plans to relocate to a central London property with a swimming pool and tennis court (for him and his YouTube gang, The Sidemen).

Olatunji's popularity is not limited to these shores, either. He was recently voted the fourth most influential figure among American teenagers in a survey commissioned by Variety, ranking above Hollywood mainstays Jennifer Lawrence and Leonardo Di Caprio, and pop stars Katy Perry and Beyonce.

YouTube’s global accessibility means Olatunji and others like him are able to reach many millions more than if they were confined to domestic television.

“I’m not really interested in TV," he says.

"YouTube has become humongous and beyond anything I could ever imagine.”