It’s unusual for a solo star to thrive in the cutthroat world of Korean pop music. Now a $5 billion global industry, K-pop, as its passionate followers across the world call it, is most recognizable for its girl and boy groups who train for up to 10 years before ever setting foot on stage. These groups, sometimes made up of more than eight people, accompany their songs with elaborate choreography, making it hard to stand out in the fierce competition as a solo artist.

Except if you’re IU.

Quartz crunched the numbers, and with 14 digital number one hits on the Gaon Music Chart, the 25-year-old megastar IU (pronounced “eye-you”) has topped the Korean chart more than any other artist since 2010. She’s outranked BTS, Girls’ Generation, and Big Bang, which are some of the best-selling K-pop groups of all time.

A large part of IU’s success is based on her pure image. Korean culture prizes wholesome reputations among celebrities, and scandals are harshly frowned upon. IU has never had a misstep. Her pristine behavior, as Koreans see it, earned her the nickname 국민 여동생, or Nation’s Little Sister—a title that IU herself has actually rejected.

While image helps, it is IU’s music that keeps her at the top of the charts. Her sound has changed hugely over her career. Her earlier singles, such as “Boo” or “Good Day,” are poppy and radio-friendly—”Good Day” was the song that officially skyrocketed her career and established her signature charming, whimsical style.

In 2013, IU shocked the K-pop world with a sudden switch to jazz for her album Modern Times. She experimented with more complex, fluctuating time signatures and big band sounds. Her two albums since then, Chat-shire and Palette, also showed versatility and artistic growth—unlike most K-pop stars, IU writes and produces much of her music. Songs on these latest albums use orchestral strings, acoustic guitar, and piano to convey a sense of vulnerability not often heard in K-pop.

As a songwriter, IU employs sophisticated compositional techniques that makes her music stand out in the pop scene. For example, she frequently uses deceptive cadences. If you are not familiar with music theory, here is what that means: Typically, whenever you hear the fifth note of a musical scale (called the “dominant”), the next chord will belong to the first note of the scale (the “tonic”). Most people find this satisfying to hear, as it creates a feeling of resolution. However, if instead the next note belongs to the minor sixth chord of the scale, it creates a sudden feeling of sadness, like black storm clouds rolling in on a sunny day. This is a deceptive cadence.

You can hear that here when IU switches to a deceptive cadence the second time she sings “Goodbye, goodbye to it all” (in Korean) at 3:30 in the song:

IU’s musical strategy has been working for her commercially. In contrast to her peers, she’s maintained a constant presence on the charts since 2010:

Yet unlike BTS, whose album debuted at number one in the US charts, IU hasn’t yet made it big in the US, the world’s largest music market by sales (pdf). Her lack of international success could be a language issue, as another big contributor to IU’s popularity is the quality of her lyrics. She writes many of her own lyrics—another rarity in K-pop—and they are often thoughtful and profound. It could also be that, unlike the big boy and girl groups, she doesn’t bring the mesmerizing choreography and flashy outfits that appeal to a lot of international K-pop fans.

It’s their loss. IU just might be the best K-pop has to offer. For a sampling of IU’s best songs, listen to the playlist below.