First introduced in DxD: Haremonogatari, this particular Sacred Gear is discussed even among the characters from Highschool DxD: Яe-birth, sealed away by the God Indra, deeming 'uverused' by him.
Unlike other sacred gears, Book Marker has several, possibly infinite users around the dimensions, which are dissatisfied with the reality and run of time around them, and by the simple thought of wanting to change the world 'for the better' in their own way, becomes hosts of this random Sacred Gear.
By using Book Marker, the users can insert themselves upon a certain dimension and change completely the flow of events from within.
Book Marker has no physical manifestation. Instead, it manifests itself in the shape of words, sentences, paragraphs and even numbers and symbols.
The real nature of Book Marker is unclear and cloudy, manifesting itself in several users at the same time, without distinguishing method of choosing, as long as the user has a strong feeling of dissatisfaction about certain events and wants to change it 'for the best' in their own thoughts, allowing Book Marker to manifest itself and take it's host.
A top-tier Sacred Gear, Book Marker has the power to insert it's user in a period of time of his/her desire, allowing them to change history and the natural flow of a once canon and stable timeline. Book Marker apparently also allows the user to change the personalities of every individual in the said timeline, by either making them fall in love with the user, or adopting a more hostile or pathetic stance against him/her. As so, Book Marker can also insert everything the user desires, allowing it to completely ignore canon rules or lore, as it can also Boost the user's powers to unlimited proportions, allowing them to become stronger than even Great Red and 666(Trihexa), among other things. The Sacred Gear apparently has no limits save for the users' desires.
It is unknown if Book Marker has any Balance Breaker. For being a wide-range and malleable Sacred Gear, it's powers and possibilities are almost endless. However, a certain pattern was detected by the God Indra to identify a certain dimension as 'Fake', and thus allowing the capture of users. Those patterns includes, but are not limited by:
- Individuals like Rias Gremory, Akeno Himejima, Serafall Leviathan or any other important female devils falling for a certain someone without proper reason;
- Either the dissapearance of the Original Oppai Dragon Issei Hyoudou or his once 'absurd evolution' halted for no apparent reason.
- The death of Riser Phenex or the record of his first lost in a Rating Game beforehand his match with Diehauser Belial.
- The survival of the Fallen Angel Raynare.
- The enroll of new students in Kuoh Academy that weren't previously affiliated with either the Student Council or the Occult Research Club.
- Any individuals described as: "Laid-back", "Cares for his friends", "Likes to taunt/provoke their foes" or anything in between. Bonus points if all the female students in Kuoh academy pay any positive attention to him if the possible user is male. Extra points if he's already associated with female devil nobles/rogues.
- Change of the gender of any individual(Mostly Vali Lucifer).
- Any relationships with individuals of extreme power, but that don't have any record in history(I.E.: A third heavenly Dragon, or a fourth Super Devil.)
Book Marker has several users. As noted by Indra and Ryuusei, the most known are:
- Ryuusei Amano
- K****i H*****u
- L******t A*******s D* L*c
- A******i G***a
- S****u F******a
- A*****a S****u
- L****l M*****e
- I*****u L*****r
- Y***i L*****r
- R***e A******i
- The image for this section is based on the character Kumagawa Misogi of the Medaka Box series.
- The names of the users were deliberately scratched by Indra in order to avoid any violence to those individuals in the original timeline.