On Wednesday, Marshall shared a touching tribute to her late costar on Instagram, featuring photos of them together on the set of the long-running family drama series.
As fans may know, Marshall plays protagonist Amy Fleming on the show while Cormier played Finn Cotter, a character that was introduced in season 15 as a potential love interest for Amy.
"Robert was truly a joy to be around. He had an infectious smile, and such a big heart. He was especially kind and gentle with the animals on set, which goes a long way in my books," Marshall, 34, wrote in the caption of her post.
"Robert, I am so thankful I was able to get to know you and work alongside you. You left us far too soon, but I promise you will always be remembered. #RIP" she added.
"Robbie was loved by many and his unexpected death has left us devastated," the family said in a statement to PEOPLE on Wednesday. "While we are broken-hearted, we take comfort in the overwhelming love and support shown to our family by Robbie's friends, colleagues and fans."
"Robbie passed away as the result of a tragic accident," the family continued. "Our family would like to thank everyone at the Critical Care team in Toronto for the compassionate care they provided Robbie and the kindness they showed our family. We will be forever grateful."
The family also asked for privacy as they continue to grieve Cormier's death.
In his obituary, Cormier was remembered as "an athlete, an actor, and a great brother."
"He had a passion for helping others and was always looking to achieve more. He enjoyed movie nights with his family, and looked up greatly to his father," the obituary read. "He impacted many people throughout his life whether it was family, teammates and friends."
The obituary continued, "Rob's memory will live on through his passion for art and film; as well as his three sisters who meant the world to him."
RELATED VIDEO: 'Heartland' Actor Robert Cormier Dies at Age 33: 'He Was Always Looking to Achieve More'
Following the news, Heartland's official Instagram account paid tribute to the late star.
"We are deeply saddened to learn of the passing of Robert Cormier. He was a beloved member of the Heartland cast the last two seasons," the show wrote in their statement on Instagram. "On behalf of the Heartland cast and crew, our thoughts are with him and his family, who have asked for privacy during this difficult time."
UPtv — which airs Heartland — also remembered Cormier on Instagram. "Robert Cormier, aka Heartland's Finn, was an amazing talent, gone too soon," the network wrote alongside a photo of the actor with Mitchell.
"We are deeply saddened to learn of his passing. Our sincere condolences go out to his friends, family, and co-workers. 🕊️" they added.
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Prior to Heartland, Cormier played Kit Jenning on the Netflix series Slasher. He also had guest appearances on Designated Survivor, Ransom and American Gods, among other television roles, according to his IMDb page. His film credits included Firecrackers, Pyrenees, as well as his upcoming projects, The Intersection, Transmission: Vol. 1 and a short called The Antagonist.
Cormier is expected to be featured on Heartland's 16th season, which is set to premiere on Oct. 2.