Grimoire Rubrum is the sentient Red Book that Nier faced in the library of Emil's Mansion. It is a complementary book to the Grimoires Weiss and Noir and its purpose is to support them. Unlike them though it's not programmed to be capable of speech.
And Then There Were None
Like Grimoire Weiss and Grimoire Noir, Grimoire Rubrum is a Gestalt bound into a book. She was originally a Crusader for the Hamelin Organization (an organization that trained child soldiers to fight the Legions with magic) and had met Weiss before as a human. She and Weiss, along with 11 other Crusader children were locked into a room and made to fight each other with magic. Those who fell were then turned into Grimoires. Weiss and Rubrum tried to stay out of the way but in the end they fell and became Grimoire Weiss and Grimoire Rubrum. The last standing was Grimoire Noir.
- Rubrum is the Latin word for "red".