She's been vegan her whole life, loves the rain and gets more excited about things as small as sparkling water than she does about her own pop-star success.
"Even if I suddenly become like the biggest thing in the world - which I don't think is gonna happen, but if that happened - I would be like, 'oh f***, that's weird. It's hard to think of it as you, you know? I think I've got big in a way, in a certain section of LA, but I don't get it."
When we meet on a dreary Auckland day, she's covered her small frame in an oversized coat and baggy ski pants in bright orange and blue.
There's no avoiding talking about her look, because between the silver hair and strikingly bright eyes it's the thing about Eilish that stands out most.
"I don't know, dude," she says, with her unique LA swagger. "I always wear like, huge pants, big jackets and like 800 layers and it's way too hot and people are just like, 'what the f*** are you doing?'
"You know what? It's just a look. I just do it. Who am I? I don't even know. I'm just so dumb," she laughs again.
But what she's doing is actually pretty smart; the LA teen makes it her business to personally cut and sew her own fashion in a bid to stand out - and it works.
"I try to make it so it doesn't look like anything else, because I want to dress like what you've never seen before. Like if you wanted to really wear what I was wearing you'd have to go here and get this and cut it in half and sew something else on to it. I wear stuff that's kind of impossible to think of," she says.
And the same goes for music. Eilish has blown up recently with the release of her EP Don't Smile At Me; an eight-track offering that mixes hip-hop-inspired production with sweet pop vocals and edgy dark lyrics to create a sound that - like her fashion - no one else is making.
But she's not doing it alone. Her older brother, Finneas, is just as remarkable: aged only 19, he produced Eilish's entire EP on his own. The most impressive part? The pair only started making music together at the beginning of 2015.
"We were both just writing a tonne and we were like, 'why don't we just do this together?' We live three feet away from each other, let's just do it," Eilish says.
Before that, Eilish sang with the Los Angeles Children's Choir, where she says she learned most of her vocal skill.
"That's where a lot of my technique comes from and my range and why I actually know my range. But I don't really know how I [sing how I do] - and I don't want that to sound like, 'oh I don't know how I do it, I'm just so amazing', I literally don't know how it works. It's really weird."
But it's not just the sonic side of things that are impressive; lyrically, Eilish is far beyond what you'd expect from someone her age, writing about everything from personal relationships and her struggle with depression to a fictional story from the point of view of a serial killer on Bellyache.
"People think you have to go through something to write about it and you absolutely do not. You can write about like, a shoe. It's a story. It's putting yourself where you wouldn't normally be, in somebody else's shoes - which nobody does, because they're just thinking about themselves. It's like, maybe think about how they're feeling and write a song from their thoughts and maybe improve yourself," she says, without realising just how profound that really is.
Between the dark pop sound, simple-yet-insightful lyrics, unique style and self-assured attitude, it's hard not to compare Eilish to New Zealand's own Lorde.
Usually, when you do that, people are quick to insist they're carving their own path.
"Dude! That's cool. Lorde? What the hell? Me? Listen, I remember when Royals came out, my mum called me and my brother and she was like, 'There's this song on the radio and it's really good and I think it's gonna be really big'. And we were like, 'Yeah okay, sure, fine'. But then it was," she says.
"Since then, she's just been so killer. That's so cool that people are like, 'If you like Lorde, you'll like Billie'. That's crazy."
But although she's happy to be in such good company, Eilish has no intention of staying in one lane for long.
"I really like hip-hop and rap, that's my main influence. I really wanna be more of a hip-hop artist," she says.
"I don't see myself as a pop artist. Like when you hear 'pop' you're like, 'oh bubblegum, jumpy little girly stuff' and I feel like, 'uh-uh, that's not me'."
Much of Eilish's sound is already steeped in hip-hop and many have described her as "alternative rap", but she wants to lean into it even further.
"Don't worry, that doesn't mean I'm gonna start rapping. But I think hip-hop production is really cool. What inspires me about rap is that it's written in an almost poetic way. I just think it's so cool. Hip-hop is definitely where I'm going."
Until then, Eilish's star continues to rise off the back of Don't Smile At Me, and she and Finneas will soon return to New Zealand to play Laneway this summer.
They recently played a sell-out show at the Tuning Fork, at which fans sang every lyric, held homemade signs and declared their love for the singer, but Eilish says next time, we can expect something a little different.
"Expect the unexpected," she says, waving her hands melodramatically and mocking herself.
"Nah, I just try to be as hype as I can on stage and have a lot of fun. My goal is to get people moshing in the crowd, one day. My music isn't there yet and people aren't there yet, but I love moshing and being in the pit for every rap concert I go to.
