Ghost Quartz

They work as a librarian and take part in patrol duty. They are also in charge of long-term recovery facilities located on the fourth floor.

Ghost's weapon of choice is a sickle.

Quiet and calm, Ghost is very insecure about themselves, thinking that their fallen partner, Lapis, was much better than them. Because of their double structure, two personalities inhabit their body, the other being Cairngorm who's very different from Ghost.

Lapis Lazuli was Ghost's former teammate. The library was put into Ghost's responsibility after Lapis was taken by the Moon People—leaving only their head.

[Spoiler] They temporarily teamed-up with Phos, but got taken to the moon after their first fight. However, Cairngorm wasn't taken with them. [/Spoiler]

(Source: Houseki no Kuni Wikia)