Ban Cheon-ak can travel between Heaven and Hell and occasionally serves as a Grim Reaper.

Gro gained the ability to see ghosts after the Key to the Celestial Realm accidentally got lodged in his head(?). While working a part-time job, his owner didn't realize he was trying to serve ghost customers, so the owner pulled his battery and almost sent him to the junkyard. Yoo Seung-chan saw him and kicked him, saying "Serves you right, lol," but then worried he went too far and tried to pick him up. While doing so, Seung-chan touched the screen—unbeknownst to him, a ghost had put the battery back in. Gro then started following Seung-chan around calling him "Master," which was their first meeting. When Gro asked for a name, he suggested Seung-chan's girlfriend could name him, but she turned out to be a ghost. He was eventually named Gro, which is short for "Gummy Robot" (Wait, no... it's short for "Beggar-like Robot"...?). He jokes around with Yoo Seung-chan a lot and is great at arguing. He is incredibly cheeky. He functions as a ghost detector(?) and has abilities like extending his arms, electric shocks, recording, filming, and voice modulation. He knows that injecting exactly 140,000 volts into a ghost will exorcise it. The Key to the Celestial Realm attracts ghosts, but since it only attracts harmless or kind ghosts, it's mostly just annoying.

There are many of them in the world. Some are harmless, while others are extremely dangerous. They are only visible to Yoo Seung-chan and Gro.