Ben said Hae Min disappeared "like a smoke". Greenhouse wasn't actual greenhouse. Greenhouse is lonely, abandoned "in the field". Greenhouses he was talking about was girls he met. Girls who want to die, Ben said they like waiting for me to burn them. In korean, "makeup" and "burn" are spelt the same way. Ben had makeup tools and make it for girls before burn them. Btw, Hae Min may not Jong su's actual former friend, she just created new friend, he couldn't recognize her.