The Agate Links was a necklace displayed in the Art Museum. It is an angelic necklace that has two wings and two blue gems. It first appeared in episode 8.
As revealed by Daiki Niwa, the Agate Links was a Niwa Family treasure (though he was uncertain about this). Forty years ago, Daiki took up the responsiblity of becoming Dark and did the family tradition, though he had no passion of doing it as he saw it as simply obeying the tradition. After doing his job, Daiki was attracted to a window that was always lit at night. It was there when he met Kurashina Menou, a blond, shy and frail girl. Daiki visited her always after his work as a phantom thief. Daiki wanted to see her smile, possibly he fell in love with Kurashina. Daiki would always make her smile and spend time together. Daiki wanted to give something to Kurashina, so he left the Agate Links in her window and flew off while she was asleep. Unfortunately, Kurashina died to following day. Her spirit have been bounded with the Agate Links, causing her to be caged inside. This explains why Saehara captured a photo of the Agate Links with Kurashina in the background (in which Saehara falls in love with her). Kurashina was waiting for Daiki for forty years, entrapped inside the necklace, displayed in the museum.
When Daiki visited the museum and saw the Agate Links, he found out that there something unusual with the necklace and requests Daisuke and Dark to destroy the Agate Links, specifically to free Kurashina from the curse of the necklace. When Dark went to steal the necklace, Kurashina appeared and tells Dark to wait for one more day, in which Dark agrees, much to his playboy attitude. When the promised day arrives, Saehara fought valiantly against Dark to protect the Agate Links (due to his love with Kurashina). Saehara fails and falls off a roof, breaking the Agate Links in the process. When Dark uses his black feather to retrieve the necklace as promises, Daisuke switches with Dark and tells him that they cannot let things end that way and gives the broken necklace to her. When Kurashina saw Daisuke, tears fell from her eyes as he saw Daiki once more, in the body of Daisuke. This was due to Daisuke's resemblance to the fourteen-year-old Daiki who Kurashina remembers. Her wish was granted and finally said thank you to Daiki through Daisuke. Kurashina ascends to the sky and happily disappears, along with the Agate Links.
Daiki reveals to his grandson that he cannot say goodbye to Kurashina personally, as she remembers the young Daiki forty years ago. That is why Daiki sent the warning note and Daisuke to see Kurashina, one last time. Kurashina also had to ask Dark to wait one more day because she was still hoping to see Daiki.
Saehara was convinced the following day that he fought Dark (which he truly did) and protected Kurashina, and it wasn't Dark that stole the Agate Links but Kurashina, who moves away with it (which truly is what happened).