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As she prepares to release her fourth EP and play her biggest London show on a headline UK tour, it’s not so much that Amber Bain is becoming bigger, as slowly coming into focus. The songs, once prettily vague and drifting, are getting brighter and catchier, and their singer is ready to be seen too.

The songwriter-producer began her music career so ambiguously — no photos of her face, taking the unusual stage name The Japanese House after a memorable childhood trip to a Cornish holiday home, cloaking her low singing voice in layer upon layer of harmonies — that some people thought she was actually a sneaky side-project by Matty Healy of widely adored pop band

The 1975. There were clues: she had been signed to the group’s record label, Dirty Hit, and worked on her first songs with Healy and drummer George Daniel.

There are certainly similarities with some of The 1975’s slower songs in the way she presents her music — an echoing guitar underpinning washes of synths, those piled-on vocals almost having a robotic effect. “But I think I was doing it first,” she insists. “I’ve been putting those harmony parts on my music since I was 14.” In any case, now that she’s revealed herself as a 21-year-old from Buckinghamshire and toured the world’s arenas as the bigger band’s support act, The 1975’s fans are on her team anyway.

A line of them, mostly studenty young girls in studiously indie attire — ripped, ragged and striped — gathers at teatime outside the Oxford venue where, in an upstairs dressing room, Bain is making various attempts to get rid of a cough in time for her show. She got back from a European tour two days previously, had one day off to spend in London with her girlfriend, the equally accomplished singer-songwriter Marika Hackman, and all the late nights caught up with her. For true accuracy, imagine every quote recounted here followed by an explosion of spluttering. She’s drinking a delightful sounding tea called Throat Coat. “I don’t know what I’m gonna do later. Just get drunk. I won’t cancel,” she says, then coughs again.

Hopefully she’ll be back to full strength by the time she arrives in Camden next Wednesday, for her music is more powerful than ever. On her most recent EP, Swim Against the Tide, released last November, she maintains a beautiful haziness to her sound, but the melodies are getting easier to grasp. “It does get a bit more bright and clear, especially the production. I think I’m getting better,” she says. The brief collection also finds room for a sparkling pop chorus on Face Like Thunder. It’s a great song, her most upbeat moment to date, and earned the distinction of being played twice in one night at her concert in Prague a week ago.

“It was my first ever encore,” she explains. “We don’t normally do them but they were screaming, ‘One more song!’ for about 10 minutes. We didn’t have another to play so we did Face Like Thunder again.”

An even newer song, Saw You in a Dream, appeared on streaming services a fortnight ago before it takes its place as the lead track on her next EP, due in a month’s time. It’s her most direct piece of music, a slow-moving guitar ballad that’s plainly about the sadness of a finished relationship. “It’s a very blatant song, not hiding behind any fancy synths,” she says. “It’s simple, quite classically written I think, which made me quite proud. I’d been listening to loads of Burt Bacharach and ELO. I thought I should just write a song that’s really poppy and easy for everyone to understand, rather than being really clever and doing five million key changes.”

If you like it, there’s only one person to credit. She says now that she’s unsure whether it was a hindrance as well as a help to be so closely linked with The 1975 in the beginning. “I always wonder whether it was a good idea or a bad idea to tell anyone that it was me, George and Matty working together. It caused this undying connection, and it affected the amount of credit I was given for production. I do a lot of production, but most people just assume that George did it.”

She recounts an early meeting, when she was putting demos on the internet, with “a very famous manager whose name I won’t mention”. He thought that being a woman who produces her own songs was a unique selling point. “He said, ‘Wow, it’s so cool that you can produce and you’re a girl, it’s so weird.’ I was 17 so at the time I thought I must be special. Now I realise that of course girls can produce. Why wouldn’t they?”

She’s had plenty of practice to get to this point. Taught to play guitar by her father as a young girl, she avoided the usual route of learning the classics and went straight into writing her own songs. Early on, she wrote one called Teenage Life, despite being 10. She asks me if I’ve seen the film Confessions of a Teenage Drama Queen starring Lindsay Lohan, as that’s what inspired her lyrics. Sadly not.

“There are so many embarrassing recordings on my dad’s computer, and a few cute little numbers,” she says. “It all sounded very like Avril Lavigne. I was super into her. I don’t know if I was in love with her, or wanted to be her, or wanted her to adopt me. It was kind of all three.”

Today she’s abandoned teeny punk in favour of something much more impressive. There’s a clear development in her sound across her handful of releases to date, though she’s still unsure what her debut album will sound like. “I’m listening to a lot more guitar music now,” she says, if that’s a hint. “I didn’t fall out of love with it, but I preferred electronic music for the last two years or so. There’s a larger amount of organic instruments on my songs now.”

After this UK tour is over, she’s planning to work on her album on weekdays during a long spell of weekend festival appearances. Early days of stage fright and deliberate anonymity are well behind her. “I was really bad at the beginning. I wouldn’t be able to speak to you now. I’d be shaking, I couldn’t really eat. Now I don’t really care.” She’s got a new backing band that includes one of her best friends from school, her stage name ensures that her tagged Instagram feed is mostly pictures of sushi, and life’s good. Cough permitting, it’s going to be a great summer.

The Japanese House plays Koko, NW1 on May 17.