I have a favor to ask. Take the notebook with you when you go. That's not all. I want you to put something in the book and send it back.
When Noctis was eight years old, he visited Tenebrae and met Luna, the young daughter of the Oracle. The two quickly befriended and Luna showed Noctis her notebook with a pressed sylleblossom inside. After the Niflheim Empire attacked Tenebrae, Noctis and Luna became estranged, but would keep in contact via the notebook that was delivered between them by Luna's magical dog Umbra. When a 20-year-old Noctis departs on his road trip to Altissia to wed Luna, he meets Umbra at an outpost and sends a message to Luna to let her know he is on his way.
Things do not unfold as planned. Insomnia is destroyed by the empire and Noctis starts his journey to reclaim his kingdom by collecting the royal arms of his forebears and forging covenants with the Astrals with Luna's faraway help. He periodically meets Umbra and learns through the notebook that Luna supports him and is looking forward to reuniting with him.
After his battle against the Leviathan, Noctis awakens days later to discover Luna has died. He inherits her notebook, and finds Luna's final message to him is another pressed sylleblossom. Noctis keeps the notebook as a keepsake.
Whenever Umbra delivers the notebook to Noctis, the player can choose the message to send to Luna. Depending on the player's responses, the item description for when Noctis will keep the book changes. The item will be called "Lovers' Notebook" in the key item menu.
If the player gives the bad answer every time, it will say "there's a trace of dust" on the book when viewed as a key item. If the player is nice, it will state the book is "well worn." If the player's responses are favorable, it will say "A notebook once used to keep in touch with Lunafreya. The last page is stained with tears."
Square Enix Cafe sells a blank notebook of the Lovers' Notebook design.
- In Platinum Demo – Final Fantasy XV, the Lovers' Notebook is among the pile of books in the room in Altissia.
- On 2017 February's ATR, Tabata explained that the reason why Noctis and Luna don't communicate through cellphones is because phones only exist in Insomnia.