Recently, Jennifer Foyle got a two-page, handwritten letter from an Aerie customer. It was complete with photos, and thanks for what the lingerie and apparel company has done for her body image.
"That's something that happens every day in this brand and that is what makes us so special," Foyle, Aerie's global brand president, told CNBC recently.
Long before the term "body positive" became fashionable, Aerie, which is owned by teen retailer American Eagle Outfitters, has been a forerunner in promoting the visibility of women with a range of shapes and sizes.
While major magazines like Sports Illustrated began featuring plus-size models in 2016, Aerie began its no airbrushing, "Aerie Real" campaign, using regular-looking models in 2014.
Female shoppers can also upload selfies of themselves — and all their flaws — using the hashtag "AerieReal," Foyle said. The campaign represents a stark contrast from other lingerie brands that showcase near-flawless models.
"Now everyone is kind of doing it because it's the trendy thing to do and it's cool to be body-accepting," said Janine Stichter, an equity analyst at Jefferies, who tracks the intimates market.
Whereas Aerie was the first, Stichter said, "it comes off as more authentic."
As both American Eagle and Aerie continue to grow — the company's stock is up 114 percent year over year — Aerie is growing at a much faster pace. The company's same-store sales growth in the first quarter rose 38 percent, on top of a 25 percent increase in 2017.
Last year, Aerie was valued at $500 million, up from $200 million in 2017. The company is gunning for a $1 billion valuation over the next few years.
A combination of the right products and successful social media branding have helped the company get to this point. In 2015, Aerie was one of the first companies to introduce the bralette, a bra without an underwire. Victoria's Secret, known for its push-up bras, was late to embrace the style.
"And when they did, it was really too late and the trend was over," Stichter said. "I think that shows that Aerie is more in-tuned with what the market is looking for."
In addition, American Eagle's kid sister has successfully expanded its athleisure, swimwear and lounge categories. Leggings from Aerie's "Chill.Play.Move" line, launched in 2016, have been a hit, Foyle said.
"The interesting thing about the company, even though they're growing quickly, they're still very small relative to the total market," Stichter said of Aerie, which has only been around for a little more than a decade. "There's still so much runway."
In fact, Stichter estimates that the intimates apparel market is worth roughly $14 billion, but Aerie's market share is still in the low single digit percentage points.
Meanwhile, long-time market leader Victoria's Secret has nearly 20 percent of the underwear and bra market, according to Euromonitor. However, that's down from a high of more than 22 percent in the last year — and the company is losing market share fast.
A spokesperson for Victoria's Secret, owned by parent company L Brands, insisted the company is still the industry leader for understanding the needs of its customers.
"Victoria's Secret is a brand that serves women, provides leadership in personal bra fit solutions and empowers her to choose her version of sexy," the spokesperson told CNBC in an email.
While Aerie continues to gain market share, smaller lingerie startups like ThirdLove, LIVELY and True & Co. are also trying to take some of the market.
"And that's the reason why the competition is important," Stichter said. "For a long time, [Victoria's Secret] was the only player with significant scale in the intimates market. Now you see a competitor like Aerie that's becoming sizable and a real threat."
Currently, Aerie has a significant store presence, that is two or more retail shops, in only 15 states, primarily on the East Coast. The stores include stand-alone stores and "side-by-sides" —locations that have either an American Eagle and an Aerie store next to each other. That helps encourages customers to shop at the sister store.
In addition, about 40 percent of the business is online, Foyle said. Aerie continues to promote its digital presence with high profile role models: British model Iskra Lawrence, gymnast Aly Raisman, actress Yara Shahidi and singer Rachel Platten. All four women have significant social media followings — about 9 million combined on Instagram — and promote Aerie on their platforms.
"They're serving as the brand voice," Stichter said, who said the role models resonate with Aerie's target demographic: Real women.
So how exactly does one define a "real woman"?
"You can see their stretch marks," said Tatiana Clark, a stay-at-home mom in northern California, who shops at Aerie. "I like that they use models of all shapes and sizes."
Lucy Huang, a 28 year-old New York City resident, pointed out that even Victoria's Secret stores are dark inside, compared with Aerie stores which are typically well-lit.
"Victoria's Secret is super hyper-sexualized," said Huang, who has been shopping at Aerie since it opened. "Aerie is just like, this is underwear. We all have to wear it."