Jamie McCarthy / WireImage 1 / 6
Jamie McCarthy / WireImage 1 / 6

By Kat Giantis

Don't be surprised if Mary-Kate Olsen starts wearing studded cow hair high-tops under her de rigueur hobo-inspired haute couture.

Both Us Weekly and Star are reporting that the fashion-focused mini-mogul, 24, is seeing New York-based footwear designer Seth Campbell, who founded the retro-looking, celebrity-beloved UES sneaker label.

The pair were spied making out "all night" at a mutual pal's birthday bash at Manhattan hot spot Lavo on Feb. 16.

"Their relationship is fairly new," an insider tells Us. "They've been able to have a few dates on the down-low. He's a downtown kind of guy, likes to party every now and then."

According to Star (via Perez), Ashley's twin sis has "fallen hard" for Campbell, although he's supposedly "a bit overwhelmed about moving so fast -- but he's very interested."

RELATED: Is Ashley's beau Justin Bartha ready to propose?

MK called it quits with artist Nate Lowman this time last year after two years of low-key togetherness, and she apparently appreciates that Campbell doesn't seek out the spotlight.

"His company is in the press but he's not, so it works out that way," the spy explains to Us. "They're both like each other and are just seeing where things go. Having fun."

The designing duo have plenty in common. Mary-Kate's The Row and Elizabeth and James fashion collections with Ashley are a favorite among celebs (Heidi Klum and Leighton Meester are fans), while Campbell's kicks have graced the feet of everyone from Diddy to Gwen Stefani.

Something else they share: a close bond with their siblings. Campbell has his sister's initials tattooed on his arm.

RELATED VIDEO: Mary-Kate and Ashley find what's new in fashion

Read more Hot Gossip