KSI runs a media empire out of his bedroom.
As the 20 million people who subscribe to his main YouTube Channel will know, that room is a humble, plain white box, somewhere in central London.
The floor is often strewn with clothes, cereal bowls and packing boxes. The blinds are semi-permanently drawn. The only clues to his wealth are a framed Pele shirt on the wall and, since June this year, a silver disc, in recognition of his burgeoning music career.
But that room has made him one of the internet's biggest stars. It's where he's filmed himself playing video games, reacting to memes, and practising raps. And it's where, over the last 12 years, he's amassed a £12m fortune, with the Sunday Times rich list naming him One To Watch earlier this year.
KSI's real name is Olajide Olayinka Williams Olatunji, although friends call him JJ. He was born in 1993 to a Nigerian immigrant couple. His mother worked as a PA, his father managed a bingo hall, but they saved enough to send him to Berkhamsted School - where the yearly fees are currently £21,000 per pupil.
Initially he did well, scoring three As, five Bs and a C in his GCSEs. He made up raps to help him study, some of which he recorded and uploaded to his first, and now defunct YouTube channel.
"The current is the flow of electrons," he sings on the perfunctorily-titled Physics Rap. On another track, he breaks down the Haber process, in an attempt to cram for a Chemistry test.
"Oh God, man, those bars were cringey," says KSI, "but at the time I thought I was sick."
The videos can still be tracked down with a bit of creative Googling - but KSI would rather you didn't bother.
"I'm trying to delete them from the internet - but it's annoying because I don't know the password or the email to that YouTube channel," he laughs. "So I can't delete it, and it's just there."
With his GCSEs over, Olatunji registered what has become his main YouTube channel - giving himself the name KSI because it stands for knowledge, strength, integrity.
Those early videos mostly consist of him playing Fifa and Call Of Duty - but his easy-going personality and boundless enthusiasm were eminently watchable. Within two years, he was making £1,500 a month - more than some of his teachers - but his grades started to fall.
"It was one of those things where you have to pick one or the other and I picked YouTube, which at the time, my parents were heavily against."
Still hoping he'd become a lawyer or a doctor - "typical African parent things" - they tried hiding his XBox and cutting off his internet.
As it turns out, they needn't have worried. KSI may not have applied himself at school, but he knew the value of hard work.
"I think working hard beats talent 100% of the time," he told Louis Theroux's Grounded podcast earlier this year.
"I've always said that I'm not talented… I just work hard to the point which allows me to reach a level similar to people that would be talented, and then break beyond that."
Certainly, his work rate is prodigious. He posts 15 to 20 new videos a month on his secondary channel, JJ Olatunji, reserving the main account for music.
Most of his videos are harmless fun - filming his reaction to funny YouTube videos, or answering questions from fans ("who is your barber?" asked one).
But it wasn't always so innocent. In 2013, he was accused of verbally abusing staff at a gaming convention. He also lost an endorsement deal with Microsoft after posting a series of "rape face" videos, in which he'd turn to the camera with a leery, suggestive expression.
He subsequently apologised and deleted those videos, saying he's "disgusted" by them. In a recent Q&A, a fan accused him of bowing to pressure from "snowflakes" and "cancel culture", and urged him to go back to his old ways.
"I don't care about cancel culture," he responded. "But the idea of me doing those old-style videos? Nah."
Instead, KSI started focusing on another outlet - boxing. In 2018, he challenged fellow YouTube star Logan Paul to an amateur bout, which became the biggest white-collar match in history, streamed by more than 20 million people worldwide.
It ended in a draw, so the two men met for a rematch in Los Angeles last November. When KSI won, he took home a seven-figure pay packet.
More importantly, perhaps, he chose to enter the ring to one of his own songs - a bone-chilling hip-hop track called Down Like That, which featured US rappers Rick Ross and Lil Baby.
In the wake of his victory, the song climbed the UK charts, giving KSI his first top 10 hit. Not bad, considering his previous nine singles missed the Top 40 altogether.
He's since followed it up with two further top 10 hits; while his debut album, Dissimulation, was only held off number one by The 1975 this March.
The success is proof, he says, that he's improving all the time - although having to do that in the full glare of the spotlight has its pros and cons.
"A big con is a lot of people get to see the trash - and they judge me on that," he says. "But a pro is that people see my progression. They see that I'm not just hopping on the trend, and that music's always been my passion and they respect it a lot more because of it."
In fact, his followers are the best barometer for his new music.
"If I make a terrible song, they'll let me know. They'll dislike those videos, pile on me, make memes of me for producing such trash music," he says.
"But then if I could produce something good, they'll let me know that, too. And that's why I'm able to get those type of songs in the charts."
His latest single, released last Friday, has so far got a universal thumbs up. Really Love is a throwback garage track featuring the vocal talents of Craig David.
It would have been a perfect summer holiday anthem, if 2020 had allowed anyone a summer holiday - but KSI doesn't care about that. He's just stoked to be on the same record as Craig David.
"Twenty years ago when I was in the car with my parents, we'd listen to Seven Days and Can You Fill Me In.... So I'm like, 'How have I ended up here?'"
He proudly shows off the star's phone number in his address book - saying the two have bonded since meeting up to record the track.
"We just clicked like, you know? Certain people that I've worked with, boy, I've really had to force it - but with Craig, it's so easy," he says. "He's the nicest guy and a lot of fun to work with."
The song has already taken up prime position on various New Music playlists, and it topped the iTunes chart first thing on Friday morning. But as KSI's music career takes off, he promises not to get swept away by fame.
"It's crazy, like, I don't even go into studios," he says. "A lot of times, I just play the beat over and over and actually just write in my bedroom."
Really Love by KSI, Craig David and Digital Farm Animals is out now.