A pet owner who lost her beloved dog of 12 years has captured the heartbreaking moment her cat appeared to mourn the passing of his "brother."

Bree W. from Brainerd, Minnesota, adopted her dog, Barney, soon after moving out of her parents' house to start college. "He was my soul dog," Bree told Newsweek. "I saw this picture of him and roommate and I went to pick him up the next day, the day after my 19th birthday."

Barney, who was about two years old at the time, would go on to spend 12 years with Bree doing all the things a happy dog does.

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"He loved to play fetch, go swimming, and one of his quirks was that he would always lead different people to the cabinet where the treats were," she said. "He loved table scraps, watching birds and squirrels and sunbathing near a patio door or window."

Seven years into their time together, Barney welcomed another pet into the family, a rescue cat by the name of 'Stache, called so on account of his mustache, who was adopted on the same date as Barney, but in 2019.

Though there are instances of cats and dogs clashing from time to time, the notion that they are incapable of coexisting has been largely debunked.

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A 2020 study published in the journal PLOS ONE, and based on a survey of 1,270 pet owners with both a cat and dog, found over 64 percent not only lived together harmoniously but also played together. The same study found 20.8 percent even ate alongside one another.

'Stache was around six months old when he came into Bree's care. He'd already endured an uncertain start to life. Barney seemed determined to change that and provide his new companion with the best big brother.

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"They bonded almost immediately and would often cuddle on the couch together, or playfight," Bree said. Barney taught 'Stache everything he knew, including how to play fetch.

"I would often toss a tennis ball across my apartment for Barney and for a while 'Stache would just chase after him," Bree said. "Then 'Stache started bringing hair ties and dropping them in front of me. One day on impulse I just threw the hair tie. 'Stache brought it right back! 'Stache wanted to play fetch too."

More important than fetch, Barney gave 'Stache a renewed sense of confidence.

"Most cats that I had known were always timid, shy and would often avoid people," Bree said. "I think since 'Stache grew up with Barney, he exhibits a lot of dog-like characteristics like playing fetch and greeting you at the door when you come home."

For a time, the two were as thick as thieves, but everything changed this summer when Barney was diagnosed with what was thought to be bone cancer.

"He was not responding to the treatments we had him on and there wasn't much else we could do for him," Bree said. "He was extremely weak and his quality of life was dwindling quickly. So we had to make the decision and by the time his final appointment came I could tell he was ready even if I never would be."

On September 6, Barney went to sleep for the final time. Bree was by his side as he passed on. "I wanted him to know I was with him through to the very end," she said. "I wanted him to know I was right there with him until he took his last breath." As they left, Bree prayed that Barney would send her some sign that he was at peace.

"I kind of scoffed at myself at the time but after he left us, we walked outside and it rained while the sun shined," she said. "Then as we drove away, heaven's rays surrounded the building. It was truly beautiful and it gave me some closure."

Sadly, 'Stache couldn't be there to say his final goodbyes. "He looked for Barney as soon as we came home," she said. "It wasn't until four days later when I saw him sitting on Barney's bed that he seemed to realize Barney was gone."

Bree shared heartbreaking footage of that moment to TikTok under the handle @breeseff. A week after his passing, she brought home Barney's ashes in an urn along with his collar.

"'Stache smelled and rubbed against it," she said. "He seems to be doing okay, but I can tell he misses Barney as he's tried to play fight with us instead now which he had never really done before."

Bree admits she's still struggling with an "immense" sense of grief following Barney's death. "I think of him all the time. I'm happy he's no longer in pain, but devastated by his absence," she said.

"I see him in every dog I meet, I miss him every time I have a meal I think he'd like scraps of, I miss the feeling of petting his fur, for a while it was hard to focus. It's getting better, but very slowly."

One day she might get a four-legged friend but for now she's just not ready to let go of Stache's beloved "big brother" Barney. She'll know when the time is right though.