Diamond died in February at the age of 44, three weeks after being diagnosed with cancer. Mills, 75, starred alongside him in Good Morning, Miss Bliss from 1988-89, the show that later got retooled as the hit Saved by the Bell.
"He was a really sweet kid. I'm very sad about that," Mills recently told Entertainment Weekly. "I remember [Diamond] always being sweet and really funny. I was very fond of him, as I was all of them."
The Parent Trap actress starred as Miss Bliss on the series, while Diamond, Mark-Paul Gosselaar and Lark Voorhies were among her eighth grade students. The young actors would go on to star on Saved by the Bell for four seasons with the additions of Mario Lopez, Tiffani Thiessen and Elizabeth Berkley.
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"The success of that series has been so great," Mills told EW. "I only did one season, but dear Mr. Belding [Dennis Haskins] carried it on while the kids just got bigger and bigger. It was fun, and I loved doing it. It's odd because even though I only did one season, it never did go away."
"What it did for me personally was that suddenly young kids who hadn't watched The Parent Trap or any of my Disney movies recognized me from Saved by the Bell. That one season gave me a whole new image on television," she added.
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Many of Diamond's Saved by the Bell costars paid tribute to him following the news of his death earlier this year.
In a statement to PEOPLE, Gosselaar, 47, said he was "deeply saddened" to hear of Diamond's death, calling the late actor "a true comedic genius."
"Looking back at our time working together, I will miss those raw, brilliant sparks that only he was able to produce. A pie in your face, my comrade," he said.
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"Dustin, you will be missed my man. The fragility of this life is something never to be taken for granted. Prayers for your family will continue on…," Lopez, 47, captioned a photo of the two on Instagram.
"I am deeply saddened by the news of my old co-star @realdustindiamond passing. Life is extremely fragile and it's something we should never take for granted. God speed Dustin," Thiessen, 47, wrote on her Instagram account.
Voorhies, 46, told PEOPLE "words cannot experience the grief that I am feeling right now."
"Dustin and I had a warm and special friendship, he was a caring gentleman, and my memories of him will always be cherished," she continued. "I am so very sorry he is gone. But it's exceptionally sad for his close loved ones who have do deal with his tragic death and their own great loss. My heart goes out to them at this trying time."
Berkley, 48, wrote on her Instagram: "I'm grateful to have gotten to create with Dustin when we were at the beginning of our dreams coming true. I will hold onto those sweet memories and the laughs we shared. Rest In Peace."