On Tuesday morning, fashion designer and businesswoman Kate Spade was found dead in her New York City apartment, aged just 55. It is believed the mother-of-one died by suicide, and left behind a note for her 13-year-old daughter.

The scene outside of Kate Spade's New York apartment on June 5, 2018. Image via Getty.

For many fans mourning the loss of the fashion icon, Spade's tragic death serves as a poignant reminder that professional success and the outward appearance of an exceptional life doesn't shield a person from mental illness.

According to Spade's sister Reta Saffo, Spade suffered from debilitating depression for years and refused her sister and husband's persistent suggestions to pursue medical help - fearing it would damage her 'happy-go-lucky' image in the fashion world.

Spade at a speaking event in April 2017. Image via Getty.

"This was not unexpected," Saffo wrote in an email to the Kansas City Star newspaper on Tuesday.

"She was always a very excitable little girl and I felt all the stress/pressure of her brand may have flipped the switch where she eventually became full-on manic depressive.

"I'd come so VERY close to getting her to go in for treatment... I'd spoken with them on the phone (not telling them exactly who the patient would be). They agreed to fly in and talk with her and take her with them to the treatment center.

"She was all set to go — but then chickened out by morning. I even said I (would) go with her and be a 'patient' too (she liked that idea) I said we could talk about it all — our childhood, etc. That I could help her fill in any blanks she might have...

"After numerous attempts, I finally let go," Saffo wrote.

Kate Spade and her husband Andy in April 2017. Image via Getty.

One of the last things Spade said to her sister was about her funeral. "She said... 'Reta, I know you hate funerals and don't attend them, but for me would you PLEASE come to MINE, at least. Please!' I know she perhaps had a plan, but she insisted she did not."

So who is Kate Spade - the woman behind one of the world's most recognisable brands? And how did she climb to the top of the fashion world, while carrying a searing, quiet pain?

Katherine Noel Brosnahan was born on the day before Christmas in 1962 in Kansas, Missouri. After attending an all-girls Catholic school, she graduated with a degree in journalism from Arizona State University. It was during her time at university, while working at a clothing store, that she met her future husband Andy Spade - the older brother of comedian, David Spade.

Andy and David Spade in 2015. Image via Getty.

"I was on the women's side. He was on the men's side," Kate said in an episode of NPR’s How I Built This in January of this year. "And one day, his car broke down, and he asked me for a ride home. And we really started off as really great friends."

After she graduated, Kate backpacked through Europe, and moved to New York. She worked atMademoiselle,a Conde Nast women's magazine that has since closed. While she promised Andy she would come back to Arizona, she fell in love with the pace of New York. Eventually, he joined her there, and together, with a third partner, they started a business in 1993 - Kate Spade.

"At the time, bags were too complicated," Kate toldNPR."And I really loved very simple kind of architectural shapes."

Kate Spade with her handbags in 1999. Image via Getty.

Kate Spade with her handbags in 1999. Image via Getty.

Kate Spade in her offices in 1999. Image via Getty.

"I would wear these very simple shapes, none of which were famous designers. I mean, there were no names. If someone were to say, ‘whose is that?’ I'd say, ‘I don't know, I bought it at a vintage store or it's a straw bag I got in Mexico.’"

The name Kate Spade came from Kate's first name and her partner's last name - and while most people loved the name Kate Spade New York, Kate's mother hated it.

"Honestly, she burst into flames," Kate said. "And she was like, ‘but you're not Kate Spade [and] now you'll never be Kate Spade, now you've, you know, you've jinxed it. And why would you name it Kate Spade?’ And I said, well, it's my first name, his last name, and it's like Dolce & Gabbana. And she goes, ‘who the hell is that?’"

The success of Kate Spade bags triggered a flurry of fakes. A fake Kate Spade bag (left), and a real Kate Spade (right). Image via Getty.

The early days of Kate Spade were a struggle, Kate toldNPR.She vividly recalled being at an early trade show, and not making enough money from sales to cover the booth she'd hired. In tears, she told Andy she wanted to quit.

He asked her whether she'd had any orders of the handbags placed, and she told him she had - from Barneys and Fred Segal.

Kate remembered that Andy looked at her and said, "Katie... You've got two of the best stores in America. Why are you crying? Let's not quit."

In 1994, Kate and Andy were married, and it wasn't until 1996 that Kate-Spade-the-brand started turning a profit.

Kate and Andy Spade in 2001. Image via Getty.

Kate and Andy Spade in 2000. Image via Getty.

Over the next few years, the brand grew and grew. "It was a snowball effect," Kate says. "It got a little bigger and a little bigger and a little bigger."

In 1999, Kate told The Boston Globe that the secret to Kate Spade was the balance between fashion and functionality.

"Handbags should be both. That’s what designers were forgetting," she said. "So many bags can hold a kitchen sink but they’re just big black bags."

"I was looking for something that could be less serious. More personal," she said. "I also wanted timelessness."

The signature style of Kate Spade bags - established early on - has indeed proven timeless.

A 2017 Kate Spade handbag. Image via Getty.

A 2017 Kate Spade handbag. Image via Getty.

A 2017 Kate Spade handbag. Image via Getty.

A 2017 Kate Spade bag. Image via Getty.