I will carry it with me always.
The hairpin gifted to Lunafreya Nox Fleuret appears in Kingsglaive: Final Fantasy XV and Final Fantasy XV. It is a golden hair ornament with a large round clear jewel in the middle surrounded by two "lids" of smaller round jewels that could invoke the image of an eye, or a full moon amid stars. The filigree pattern of the pin is similar to King Regis's crown.
Here. She was carrying this. It was meant for you. Keep it. She would've wanted you to have it.
The hairpin is supposedly a wedding gift from King Regis of Lucis to Lunafreya who is to marry Regis's son, Prince Noctis. The hairpin was to be delivered to her by Crowe Altius of the Kingsglaive to see Lunafreya safely to Altissia for the wedding. Things went awry however: Ravus Nox Fleuret stopped Lunafreya from leaving Fenestala Manor and Emperor Aldercapt brought her to Insomnia for the treaty-signing ceremony. Crowe was killed during her mission before she got to Lunafreya due a conspiracy of the Kingsglaive plotting to betray the Crown.
Titus Drautos passes Crowe's effects onto her friends Libertus Ostium and Nyx Ulric and reassigns the latter to guard the princess. Nyx meets Lunafreya during the party at Caelum Via to celebrate the peace treaty and gives her the hairpin. He relays the news of the original gift-giver's dire fate. Lunafreya is saddened that the Glaive entrusted to give her the gift passed, but deems the gift lovely. Titus is also at the party as his alternate identity as General Glauca to witness Nyx and Lunafreya's meeting.
Unknown to Lunafreya and Nyx, the hairpin is a tracer. Crowe had a watch to track it, though the original purpose of the tracking device is not explained, nor whether the tracing system was at the behest of King Regis (from whom the gift is said to be from) or Titus Drautos (who gave it to Crowe). Crowe's watch displays the coordinates of the hairpin, but she was killed just after departing Insomnia. Both the watch and the hairpin were then passed onto Nyx by Drautos, but Nyx seems to have thought the watch was broken due to it not displaying the correct time. When Nyx learns Lunafreya is missing following the party at the hotel rooftop, he races back to his apartment to find the watch, realizing it displays coordinates, allowing him to find where she is being held.
Your hairpin. They're tracking it. Give it to me.
Nyx rescues Lunafreya who insists they return to Insomnia to protect King Regis. Though they manage to get to him, Regis is killed by General Glauca after passing the Ring of the Lucii for Lunafreya's safekeeping. When Lunafreya and Nyx are trying to escape Insomnia, Nyx realizes the empire is now using the hairpin to track their whereabouts, intent on recovering the ring. Nyx removes the tracer from the hairpin but drops it in battle. Lunafreya picks up the tracer and tosses it into a machine that explodes, destroying the tracker with it. Whether Niflheim knew about the tracer since the beginning, or only learned about it after Nyx found Lunafreya so easily, is unknown.