It appears to be some sort of love letter. Whoever wrote this had some very strong feelings, indeed.
Letters is a Datalog segment in the Archives in the Windows and Royal Editions of Final Fantasy XV. It chronicles the letters the player has found. The letters from Ravus are found around his person in Chapter 13 and are missable if the player does not read them when they come across them as part of the story. The last letter is available from Chapter 9 onwards, by returning to Lucis (with Umbra or the royal vessel) and taking on the quest for Melusine from the Meldacio Hunter HQ; defeating her yields a faded letter.
Letter from Ravus 1Edit
I shall send an escort. You and Gentiana will return to the manor. It is dangerous to remain in Lucis—the empire fears the covenant with the Archaean, and ere long they will come for you.
Letter from Ravus 2Edit
Noctis may have the Storm's blessing, but his powers remain as feeble as a breeze. Open your eyes, Lunafreya, and see the futility of your toils. He is unworthy of your sacrifice, and of King Regis's sword.
Letter from Ravus 3Edit
Rely upon the Altissians if you must, and cease all contact with the empire. If you seek a covenant, you know full well the cost. Should Noctis succeed in claiming the power, I shall gladly restore his father's sword to him.
Can you believe we've been together for three whole years? Whoever said 'time flies' was right. And I know it hasn't always been sunshine and rainbows, but I want you to know that I care about you more than anything else in this godforsaken world. When we first met, you were like a little sister to me—someone I had to protect, but now I see you for the thoughtful and courageous woman you've beco—
The key item description is: "Love letter penned by a man who seems to struggle expressing his true feelings. The ink has faded with time, and parts of the text are no longer legible."