In the movie I watched it had an Asian kid I’m not exactly sure what he was but he had an imaginary monster friend that looked like a hamster with really big eyes. And only kids could see these monsters.
At some point the boy was on a train with them as he traveled to help them fight a battle in their deminsion.
In the end you can see the boy all grown up walking out his front door and the little hamster thing tries to get his attention but since he grew up he can’t see him anymore.