Mac DeMarco’s got a lot to be proud of at the moment. The splat of vomit on his red plimsolls, which he immediately points to as I arrive, is not one of them. “I threw up at the bar,” he says triumphantly, cigarette in one hand, coffee in the other. Standing in a grey T-shirt and boxers outside the council-block Airbnb he’s using in London, his phone shoved in his elasticated waistband, the 25-year-old Canadian doesn’t look like a countercultural phenomenon, but that’s what he has become. He has never had a single song or album in any UK charts, but in three years has climbed from 250-capacity rooms to two sold-out shows – that’s nearly 7,000 tickets – at the Roundhouse in London.

DeMarco is one of the most distinctive songwriters in modern guitar music, his songs recalling the hazy romance of a more innocent era, a sound that rouses an incongruously seething mosh pit from the DeMarco-clones at his raucous gigs. His popularity is cause for celebration, which means downing wine and shots after the first of his Roundhouse shows. “Did I barf on you?” he asks his manager. She says no, but there’s a pragmatic tone to the exchange that suggests it may have happened before.

DeMarco crawls into his bed – a sheetless mattress on the floor – to conduct the interview. Next to it sits a curry, which he’s planning to eat once his stomach has settled. So far, so skanky. But there’s a surprising earnestness to his answers. He is savvy about saving money because, in his words, “These funny little Pitchfork music careers aren’t going to last for ever,” and is quick to dispel myths surrounding his supposedly hedonistic lifestyle. He seems a little sheepish when I bring up his role in Granny (Tyler, the Creator’s ode to senior sex), and slightly bewildered when I mention a tweet about his gig the previous night, which suggests he dropped a pill before performing Metallica’s Enter Sandman.

“A pill? A pill of what? Drugs? No, I was totally sober.” He looks shocked, a little bit sad. “I did not take anything. Even if I had, I don’t think it would have kicked in in time.”

He’s a bit tired of the gossip. A trawl through the internet shows the extent of his mythology – Reddit threads speculate about his drug intake, and his relationship with his longtime girlfriend, Kiera. How does the constant scrutiny feel?

“I’m essentially an internet meme at this point, so people can think what they want, believe what they want – it doesn’t matter to me. The thing is that sometimes I am crazy – like last night. The opener, this band Happyness, I was reaching my hand down their pants and touching their penises. You know, if the mood calls for it, then it’s time to rock’n’roll. [But I am also] a human being – that’s what people forget sometimes. If I was zany all the time, then I’d be a one-trick pony but I’m a guy, I’m a human. It’s a rollercoaster.”

DeMarco started winning attention in 2012 when his album 2, was released on the hip Brooklyn indie label Captured Tracks. He quickly began to win fans, not just for his wonky music, part Jonathan Richman and part John Lennon, but for his eccentric, often juvenile showmanship. He represented something different. Because of the immediacy of his character and the intimacy of his music (2 ends with audio of him waking Kiera, who’s fallen asleep on the sofa), DeMarco’s private life has also become the property of his fans. It’s not helped that on My House by the Water, the last track on Another One, he reads out his full home address: “Stop on by,” he says. “I’ll make you a cup of coffee.” So far 400 devotees have taken him up on the offer.

“It’s weird. I think that the way that me and my band present ourselves on stage or in the press or whatever gives people – I wouldn’t say a false sense – but a sense of ‘Mac would have a beer with me!’ And it’s true, I probably would, but I’m just like the kids that listen to my music. I’m a regular, normal guy – a cheap guy living in a weird house somewhere and making some songs, but I think it’s nice to have a relationship with fans, instead of being some pretentious art weirdo [who’s] like ‘Leave me alone.’ Fuck that shit.”

Born Vernor Winfield McBriare Smith IV to a family full of musicians, DeMarco (his name was changed by his mother to McBriare Samuel Lanyon DeMarco) describes his upbringing as “pretty meagre”, with an absent dad and little cash to spare (his mum, Agnes, asked about her son by Pitchfork, said: “What do you say to a father who, you know, chose alcohol and drugs over you?”). He wasn’t interested in studying, aspiring instead to be part of Edmonton’s creative community. The alternative, after all, was pretty grim.

“Outside of that small arts community, it’s a pretty weird place to be. For example, for the past 15 years they have had tar sands up north, which is refining really dirty oil, melting the polar ice caps, and all these guys who dropped out of high school are able to go up there and make a whole shitload of money, because it’s really dangerous work. Get hookers, get addicted to meth, buy a huge house, buy a huge truck – and they’re not really concerned about what they are doing with the planet but they’re very happy to be getting a new hockey stadium next year.”

There are elements of Edmonton that remain part of him, however.

“It’s very cold, it’s very isolated and I came from a really crazy drinking culture. From an early age, me and my friends were like, ‘Let’s get fucked up!’ and it becomes this thing that’s … people consider me a binge alcoholic. If I’m going in, I’m goin’ in hard – but for only one day.”

Those hoping he is a new bastion of bad behaviour will be disappointed to read about his restraint. Perhaps it’s what upset the Fat White Family, who, in a recent Facebook post, threatened to “join Isis” if DeMarco remained in the public eye. They particularly singled out a live performance in which he brought Kiera out for a cuddle.

“People ask: ‘These are your friends, right? This is a funny thing?’” he says. “I’m like, ‘I’ve never met these guys. I don’t know what they sound like.’ They asked me to play guitar with them on David Letterman a couple of months ago and I said no. But I guess somewhere in between that time and now they decided that they didn’t like me actually and yeah, if they have a problem, then that’s fine. Not everyone’s going to like my music. That’s fine.”

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Did it hurt his feelings? He shakes his head.

“I don’t know anything about them. I don’t know anything about their music but it just kind of comes off as misinformed and a little bit disrespectful.”

Amid the body fluid and manhandling, respect and love are things DeMarco is very serious about: it’s his lasting message at each of his febrile gigs, and at today’s interview.

“Like it or not, I am essentially a role model for these kids, and you know I smoke and drink and make lewd jokes and dress sketchy and people are probably like: ‘Oh, he probably stinks bad,’ and shit. But that stuff is so surface it doesn’t really matter. The important thing to me, especially on stage, is that people come to exude some – the message of being kind, of being respectful, of being nice to everybody. Kiss your mum on her head when you go.”

He smiles, a sweet, dutiful grin. It’s time to get ready, or rather put some trousers on, for another show. Unlike the nether regions of last night’s unfortunate support band, the takeaway curry remains untouched.

Another One by Mac DeMarco is out now on Captured Tracks.