Caitlyn, whose eight-part documentary airs on E! beginning July 26, sported an airy, peach-hued dress by Michael Costello in a promo video for the series—a chic look that Kim had made for her family trip to Mexico last year.
"I originally made that dress from Kim," Costello explained. "That's why it's a little short [on Caitlyn]. She told me wanted to wear some things when she was going to Mexico. We talked on the phone and we met in person. We constantly FaceTimed about this dress and sent videos back and forth about the perfect nude [and] shade of blush."
Costello first noticed his dress resting on a couch in Caitlyn's Malibu home in the Keeping Up With the Kardashians: About Bruce special that aired in May.
"I see this fabric on the couch in the video and I'm like, ‘Is that my dress? That has to be my dress! This looks too much like it.' And then later I saw the [I Am Cait] promo video and I said ‘Oh my God, that's crazy.' I can't believe it—she's wearing my dress," Costello said.
(Last month, Caitlyn Jenner chose to identify publicly as a woman and E! News will refer to her using female pronouns. In stories published before this date, she was referred to as Bruce Jenner and male pronouns were used.)
The designer also has some expert insight on Caitlyn's style, which, as we've seen to this point, favors slim-fitting wrap dresses, slinky LBDs and printed skirt and blouse combos.
"She likes simplicity and she likes to be chic but feel effortless. I don't think she likes a lot of fuss or a lot of gizmo and gadgets in a dress, or too many zippers, or protruding hips. I think she just likes to feel sexy, confident. It's more like she wants to wear clothes that she's always wanted to wear—that make her feel good about herself," Costello said.
James White for E!
As far as what Caitlyn will wear to the ESPYs, we've made our guesses! And Costello actually submitted designs for the star to consider. (We've heard whispers that Angelina Jolie's stylist Jen Rade may be assisting with styling duties for Caitlyn's red carpet debut tonight.)
"We submitted some really pretty sketches, and we did a lot of black and strapless and we did a few royal blue colors," Costello said. "I know that they did ask for some stuff to be strapless. And we said ‘OK, well we can make some strapless options,' which I think would have worked. And they also asked for a halter option, so we did a halter option too."
"I think she's gearing toward more elegant and sophisticated silhouettes, like a nice mermaid silhouette, very fitted at the waist. If it's not something that's super fitted than something that's very flowy. She really likes things that hit women in the right areas. Her waist is really small," he added.
Whatever she wears, we're sure Caitlyn will look gorgeous! The star will be joined by members from both the Kardashian and Jenner side of the family for her big night, in which she will receive the Arthur Ashe Courage Award in front of millions.
I Am Cait premieres July 26 at 8 p.m., only on E!
PHOTOS: See more of Caitlyn Jenner's best looks