Kate was married to Spade's brother, Andy, from 1994 until her death on June 5. The two co-founded the handbag and fashion label Kate Spade New York and share one child, Frances Beatrix Spade, 13.
Spade told "GMA" anchor Robin Roberts that his family was doing “as good as we can."
"It’s been very tough, obviously," he added. "She really made an impact.”
The actor/comedian, on-hand to promote his audio book "A Polaroid Guy in a Snapchat World" and his movie "Father of the Year" dropping on Netflix July 20, recalled how a run-in with Kate would motivate his companions to dress to the nines.
“It’s funny ‘cause every girl I was with, or anyone that knew they would see Katie, would dress differently. They would bring their A-game," he said. "It was very sweet.
"They would be like, ‘Is Katie coming? Are Andy and Katie gonna be there?’" he continued. "And if they were, it was a whole new wardrobe."
Spade said Kate, being "very sweet" and knowing his friends had dressed to impress, would compliment them.
"And when I’d say, ‘You don’t dress up for me,’ they’d go, ‘Yeah, that’s her and you’re you,'" joked Spade.
Noting he could just "start bawling," Spade said of his sister-in-law: “She was such a beautiful, lovely, great spirit that obviously will be missed.”
Kate was found dead in her New York City apartment last month of an apparent suicide, Det. Sophia Mason, a spokeswoman for the NYPD, confirmed to USA TODAY. She was 55. The designer was also the aunt of actress Rachel Brosnahan, who stars in Amazon's "The Marvelous Mrs. Maisel."
Kate was laid to rest in her hometown of Kansas City on June 21.
Contributing: Cara Kelly and Maria Puente