The biggest act on Glastonbury’s Pyramid Stage this year isn’t Coldplay or Dua Lipa. In fact, you might not even have heard of them.

The act in question is Seventeen - a K-pop boyband that boasts a whopping 13 members, and which sold more than 10 million albums last year.

The only act to sell more? Taylor Swift.

But despite K-pop's global success, the UK has remained frosty towards the genre.

Even though bands like BTS, Blackpink, Stray Kids and Aespa have broken out, their biggest hits have tended to come in collaboration with Western artists like Selena Gomez, Lady Gaga or Coldplay.

Seventeen don't do collaborations. Unlike most Korean pop acts, they also write and produce their own material. Now, they're making history as the first K-pop act on Glastonbury's main stage.

The scale of the honour isn’t lost on them.

“There's a great sense of responsibility,” says Choi Seungcheol, aka band leader S Coups.

“We’ll keep coming back to that feeling and do our best to prepare, so that we can blow everybody away... Not just our fans, but every other member of the audience.”

Seventeen play the festival in a year that sees it moving further away from its rock roots than ever.

Reflecting the audience's evolving listening habits, they'll be joined on the Pyramid Stage by Afrobeat icons Burna Boy and Ayra Starr, while pop star Dua Lipa and R&B sensation SZA gain headline status.

Seventeen play shortly after lunchtime on Friday afternoon. It's a far cry from the late-night sets they play in Asia but S Coups is unfazed.

“The time of day doesn't really matter to us,” he says.

“It's actually even better that we’ll be performing in the afternoon because the audience can see us better under sunlight.”

That positivity will be familiar to anyone who’s listened to Seventeen’s music.

Lemonade fresh and full of zing, songs like Shining Diamond and Very Nice are a welcome reminder that pop can be fun.

The band’s origins lie in the pistachio green basement of South Korean entertainment company Pledis.

That's where members were selected, rejected, coached and pruned over a long and gruelling apprenticeship.

“It lasted maybe four to five years,” says rapper Jeon Won-woo, known to fans as Wonwoo.

He was one of the first recruits back in 2013 and survived several rounds of auditions, all of which were documented in the online series Seventeen TV.

The show was unscripted and unedited, often running for hours at a time. Fans on the livestream could choose the camera angles, watching as the teenage trainees ate meals, faced evaluations and, on one notorious occasion, were scolded for fooling around.

At one point, the series vanished for eight months, leaving fans wondering if the group had been dropped.

Looking back, S Coups says that hiatus – during which three members were replaced without explanation - helped Seventeen take control of their destiny.

“Our deal was pushed back a little and Woozi, one of our members, thought we could debut earlier than planned if we made our own music.”

Woozi, real name Lee Ji-hoon, turned out to be the band’s ace card.

A classically-trained musician, he has co-written and produced the majority of their output – more than 140 songs to date.

“He inspired the other members to take an interest in making their own music, so they started to learn song-writing and lyric-making, too," says S Coups.

“Being engaged in our creative process goes a long way towards making our music more authentic. so we take great pride in it."

“That’s the great weapon we have,” adds Wonwoo.

It seems improbable that Pledis would surrender creative control after four years of moulding their young recruits - but the band say the relationship is based on mutual trust.

“Sometimes we’d make something and it would come out differently than the company had planned,” says S Coups, “but I wouldn't necessarily call it a fight".

"It's more like convincing them to come over to our side and understand our vision.”

Seventeen finally made their debut in May 2015, with an hour-long live showcase on South Korean TV, followed by the release of their first EP, 17 Carat.

At that point, the line-up was settled as (deep breath) S Coups, Jeonghan, Joshua, Jun, Hoshi, Wonwoo, Woozi, DK, Mingyu, The8, Seungkwan, Vernon and Dino.

From there, they're divided into a vocal team, a hip-hop team and a dance team. The band’s name is derived from tripartite set-up: 13 people + 3 units + 1 band = 17.

S Coups, born in 1995, is the oldest member of the group and its official leader - a role he approaches with humility.

“I try to listen to what every member has to say and have everybody aligned on the same path,” he says.

“My goal is to make sure we can last as long as possible, so we can make as much music as possible.”

That’s not to say the band lacks hierarchy: Woozi takes charge of the music and Hoshi leads the choreography, but each member is encouraged to contribute.

US-born Joshua plays guitar, Seungkwan is the strongest vocalist, The8 is the resident fashionista.

As the name suggests, they're more than the sum of their parts. Each member brings a different flavour, but when they perform together, they move like a pack.

Fans have even observed that when Seventeen rehearse, the squeak of their trainers is so in sync that it sounds like one shoe, not 26.

But it's not always so smooth.

The8 (real name Xu Minghao) threw out his back in one rehearsal, Seungkwan has undergone corrective ankle surgery, and S Coups only recently returned to the stage after damaging his anterior cruciate ligament.

“I’m great now,” he smiles, acknowledging that injuries are an accepted part of the job.

“The choreo is so physically exhausting, our bones and ligaments and muscles are tired all the time,” he says.

As the band know all too well, hard work doesn’t guarantee instant results.

Seventeen’s debut EP sold just 1,400 copies in its first week and it’s taken a decade to reach the point where they can achieve millions of sales in pre-orders alone.

Along the way, their sound has progressed from the frothy pop of 2015’s Adore U to the broody, syncopated EDM of 2017’s Don’t Wanna Cry and the genre-hopping exuberance of 2020 single Left & Right.

More recently, they’ve experimented with Amapiano, deep house and distorted guitars. And last year’s mini-album FML raised eyebrows for its atypically downbeat title track.

“I'm getting so numb to this life that it feels like I'm shrinking away,” sang The8 over an old-skool hip-hop groove.

“When we were putting together the album, some of the members were going through a rough time,” explains Wonwoo. “Our mindset is reflected in the songs.”

“But when we honestly, openly talk about our feelings, it really helps to get rid of the negativity.”

As if to prove it, FML's sister single, Super, was an upbeat celebration of the band's internal strength: "I love my team, I love my crew".

Speaking through a translator in Seoul, S Coups, Wonwoo and The8 all pick Super as the perfect primer for any newcomers (Jeonghan, who also joins the Zoom call, opts for the posse cut Very Nice).

“It’s such an intense song, it really shows Seventeen’s presence,” says Wonwoo.

The two singles propelled Seventeen to new heights. The FML EP was the biggest-selling record of 2023, shifting more than 6.3 million copies and winning album of the year at the Asian Music Awards.

“I expected it to do well, but not this well,” laughs S Coups.

To get new fans up to speed, the band have just released a 33-track compilation, 17 Is Right Here, collecting all of their singles to date – as well as a new track, Maestro, that acts as a manifesto for their freewheeling sound.

“Mix and match, it's our thing,” raps Vernon. “Crazy is the new normal!"

“In a way,” says Wonwoo, “the song is us applauding ourselves for all the effort that we've put in, and the journey we’ve had to take to stand where we are right now.”

But even the world’s biggest-selling artists have goals.

Glastonbury is part of a plan to expand their audience even further – with a world tour on the cards for the end of the year.

“We’d like to reach fans in as many countries as possible, because we don't really get to see them that often in person,” says S Coups.

“I still believe that we have a long way to go and we want to become even more successful.”