I'm glad I'm not the only one who is having doubts about Officer Choi! I was on the fence until the dog violence scene, and now I'm pretty mad, even though we don't know for sure it's him. I don't remember many kdramas touching on cruelty to animals, and I'm glad that they don't.
If there is something possessing him, maybe there's a loophole so that she can't see it? Like he's higher on the ghost totem pole or he's something else entirely. Maybe a shaman lady has a bell necklace that prevents other ghosts from seeing him. Hard to say for sure this early, but it's a great theory. Either way, I'm still feel pretty disturbed about the dog violence.