The giant unnamed Robot from the 2016 film Colossal was, in reality, an avatar of an American bar owner, Oscar, which would mysteriously manifest in the city of Seoul, South Korea while he was physically in a undisclosed town in an American northeastern state.
Through some not fully explained method but which was apparently fueled by the intense anger of Oscar's childhood friend, Gloria, 25 years prior who witnessed Oscar stomping on and destroying her school project diorama of a made-up kaiju monster attacking a city. Angered by the betrayal, she was struck by a mysterious lightning-like energy which also destroyed Oscar's project which she had been holding of a giant robot attack.
25 years later, as adults and standing on the same spot, then a playground, and at the same time of morning as the incident, Gloria and Oscar discovered that they were the true source of a kaiju monster and giant robot which were appearing out of thin air in Seoul, terrorizing and causing massive destruction and loss of life which they had been watching on the news, the playground and Korean city somehow linked together. Their avatars only existed in Korea for a short specific amount of time daily, the time which the almost forgotten childhood incident lasted from start to mysterious lightning strike, a period of about five minutes.
While Gloria was grief-stricken by her role in the mysterious manifestations and sought to prevent repeating them, Oscar, jealous for her affections, sought to exercise control of her and keep her under his thumb by threatening to continue to manifest his robot avatar to cause destruction and loss of life if she did not agree to do as he required of her. In Korea, his robot would battle her monster, she trying to stop him, he wanting to cause mischief and destruction.
Eventually, Oscar was stopped by Gloria when she gambled that if she traveled to Seoul then her kaiju monster avatar might manifest at the playground. She was correct and seeing Oscar's Robot turn to flee, she caused her monster to reach down and grab his real body, pick him up, and fling him far away, the robot mimicking what happened to his body.
Oscar's death is presumed as being unable to survive such a throw, as well as the giant robot disappearing into thin air one final time.