Teenagers are already having their Billie Eilish phase. Ask anyone between the ages of 11 and 19 about her, and they will say she has been famous since 2015, when as a 14-year-old she uploaded her indie ballad Ocean Eyes, written by her older brother, to the internet. They will mention her distinctive style, a swamp of bright colours and chains, a look equal parts SoundCloud rapper and Sesame Street character. They’ll also add that she has 14.2 million followers on Instagram, and that Lana Del Rey and Dave Grohl are fans. Grohl has compared her devotees to those of Nirvana. Ask any of the hundreds of girls who have been camping outside the Shepherd’s Bush Empire in early March to see her play three consecutive sold-out dates, and they will laugh at the fact that adults don’t know who she is; that she’s their secret. They would probably say their Billie Eilish phase is different.

“People at my meet-and-greets have said: ‘Billie, I feel the way you feel about Justin Bieber about you,’ and that blows my mind. I just feel bad because if that’s actually true?” She shakes her head solemnly. “I’m sorry, bro. I do not mean to be putting anybody in the position I was in. That shit hurts.”

Bury a Friend.

From her recent eerie single, Bury a Friend, to the brooding vision of grandeur in You Should See Me in a Crown, Eilish’s music conjures a twist on dark, theatrical pop, sharing as much DNA with the broad strokes of Broadway as it does Del Rey’s haunted balladry. The release of her debut album this week may mark the first time that many parents have heard Eilish, whose music represents everything about Gen-Z pop culture that foxes adults: genre-less but image-conscious; extremely online, but private. It deals in anxiety, sincerity and emotional intelligence, mixed up with classic teenage apathy. Her music, like her style, is difficult to place on a timeline or pin to specific references. It’s new and it’s accomplished. Eilish embodies it all.

At the start of our chat in a hotel near the Shepherd’s Bush Empire, she takes off what looks like a bulletproof jacket and tosses her phone down on the sofa, checking it only once in an hour. Her hair is the same shade of blue as her eyes. A teenage cousin warned me that she seems intimidating from her withering photos. But in person, she is hugely likable, calling me and everyone else “bro” and “dude”. She recently told fans she has Tourette syndrome after people made video compilations of her rolling her eyes, which is how the condition presents for her.

And Eilish isn’t joking: she genuinely doesn’t mean to cause her fans pain. “I consciously have been the artist I would have wanted to have been a fan of growing up.” Along with Bieber, all her youthful favourites – My Chemical Romance, Avril Lavigne, the Beatles, Del Rey – have vast fanbases, where, as a fan, you’re one of millions. “I don’t want to be out of someone’s grasp. All I have tried to do is be available to fans because I never got that. I dunno …” She trails off. She cares and she doesn’t care, and, as she reminds me whenever the conversation gets serious, it’s not that deep.

The reality isn’t lost on Eilish that she is already almost famous enough to be out of her admirers’ grasp. But until now, understanding what fans want has been at the core of her rise. Initially, she refused to do meet-and-greets, deeming them “gross”. Instead, like Moses parting the waves, she would step into a crowd and spend hours hanging out with fans. She replied to every single DM, and liked every tagged picture. “If I tried to do that now?” she says, and laughs faux-maniacally.

‘Fans tend to mention her distinctive style.’ Photograph: Kenneth Cappello

Being a child of the internet, knowledge was accrued by osmosis. She learned from “the bad”. She would relish “Celebrities’ Shadiest Moments” or “Diva Moments” or “Why This Musician Is Problematic” videos on YouTube. Once, she went to a meet-and-greet at which the artist was so cold that she stopped listening to their music. “No one’s career is what I would want. No one can do this shit right,” she says, citing Rihanna’s progression as the closest example of what she aspires to.

Her efforts worked. When I ask the fans waiting outside the venue why they love Eilish so much, they repeat a word that has become almost meaningless in a time of influencers and industry plants: she’s “relatable”. Twelve-year-old Sasha says Eilish is on a level with them and that “if she was older, like Beyoncé, we couldn’t relate”. Her friend Pearl adds that Eilish’s music and fashion evolution through her teenage years seems authentic: “It’s a gradual change that a normal person would make.” Notably, all the girls are wearing baggy shirts or hoodies with skirts and trainers, a freedom they attribute to Eilish’s influence. And, for a generation that craves it, Eilish created her own fame. “It gives us this feeling that we can do it, too,” says Pearl.

Eilish grew up in Highland Park on the outskirts of Los Angeles before the area became gentrified. It was, she says, “janky as hell”. “I couldn’t go outside past dark because it was too dangerous and shit; there were gunshots … a lot. People have a really weird interpretation of how I grew up, and I think it’s because I’m a girl, I’m from LA and an artist. Automatically, people think you’re from [she puts on a Valley girl voice and snorts] Beverly Hills or some shit. Not at all. I grew up with no money at all, I grew up poor. I had one pair of shoes and a shirt.”

She was home-schooled in an environment that sounds fluid and freeing. Her parents followed her every creative whim: horse riding, dancing, piano lessons. Eilish worked shifts at the ranch in exchange for riding lessons, and says that, despite a lack of income, both parents being part-time actors (her mother, Maggie Baird, is also a screenwriter, songwriter and aerialist) meant they prioritised putting their spare cash into creative endeavours. “I’ve seen people get completely lost because they’ve been told what to do their whole lives,” Eilish says. “From a young age, I got to figure out what it is that I wanted, and not be forced to do something someone else wanted me to do.”

I’ve seen people get completely lost because they’ve been told what to do their whole lives

After describing it all, she sighs. “I miss my childhood.” Traces of it still follow her. Her mother and her brother – Finneas O’Connell, who appeared in Glee and co-writes Eilish’s songs – go almost everywhere with her. But her reality now is that she can’t escape “Billie Eilish”, the product. She was watching a “shadiest moments” video online the other day and saw herself in it. It triggered the same sense of betrayal she feels when she accidentally comes across cruel memes about her while scrolling through humour accounts. But “if someone’s being mean about me, that shit’s funny. Someone’s laughing. I don’t care. It’s not that deep”, she insists.

I believe that she thinks that’s true, but still, knowing something doesn’t matter doesn’t make you immune to feeling. Having grown up amid “cancel culture”, where public figures are ostracised for making mistakes – from a dodgy old tweet to more serious infractions – she thinks she is less likely to fall prey to it. “That shit’s scary, though.” She shivers and makes a spooky “Oooh” sound, looking me straight in the eye. “I’ve had so many nightmares about that.”

Lucid dreams, night terrors and sleep paralysis litter Eilish’s debut album, When We All Fall Asleep, Where Do We Go? It’s an album of firsts, of which Eilish has had plenty over the past year: being in love, experiencing the death of someone close and first time (she adds with another snort) being famous. She says it’s about the parallel awfulness of dreams and reality, and, occasionally, the pleasurable, dreamy quality of being alive. Her young life conveys no obvious torment or trial, but she makes a few lyrical references to her personal safety. She says she wouldn’t be able to do casual meet-and-greets any more for that reason. Although she feels happy on stage, she doesn’t feel physically safe up there. Before shows, “I’ll go through the back entrance because it’s the safest way for me to go. Sometimes, there are not-great people outside: not fans, sometimes people who … don’t want the best things for me.” Later on at the gig, I personally feel uncomfortable at the number of solitary older men incessantly taking photos of her.

She steers clear of danger where possible. One song, Xanny, refers to Xanax, a drug newly glamourised by a generation of young, disaffected SoundCloud rappers. “I have never done drugs, I’ve never got high, I’ve never smoked anything in my life. I don’t give a fuck, I never have. It’s just not interesting to me. I have other shit to do,” Eilish says. During a period where western teens have been experimenting with downer drugs, her message to young fans is clear. “I know people around you doing that shit makes you want to, but you don’t have to.” The song’s message, she says, is “less ‘don’t do drugs’; it’s more ‘be safe’”. It’s a plea that comes from her personal experiences with loss. “I don’t want my friends to die any more.”

Eilish performing last year. Photograph: Jeff Kravitz/FilmMagic

Before each show, Eilish gets driven around the block adjacent to the venue. She’ll look through the windows at the fans waiting outside – just as her fans observe her through a screen. “Even if they don’t see me, I like to see them because they’re so cute, dude.” She holds her hands up to her face to emphasise her point; large silver rings spell out “Billie” and “Eilish”.

Half an hour before the doors open, in a nearby Starbucks, a young Brazilian woman buys a coffee. “What are you here for?” the barista asks, surprised she’s come all the way to London from South America. “Billie Eilish,” she answers excitedly. He repeats the name twice with the coffee on the counter between them, eyes glazed, like speaking will conjure an image into being. Billie Eilish. Billie Eilish.