Warning: SPOILERS for Star Trek: Discovery season 3, episode 4, "Forget Me Not".
Star Trek: Discovery's Adira (Blu del Barrio) was surprised by a twist involving her late Trill boyfriend, Gray (Ian Alexander), but it was similar to what Ezri Dax (Nicole de Boer) experienced on Star Trek: Deep Space Nine. In Star Trek: Discovery season 3, episode 4, "Forget Me Not", Commander Michael Burnham (Sonequa Martin-Green) accompanied Adira to Trill so she could unlock the mystery of why she was unable to access the memories of the late Admiral Senna Tal (Kenneth Walsh). But Adira's experience on Trill, while successful, opened up another mystery regarding Gray.
Adira is a 16-year-old human who was in love with Gray Tal, a Trill who had just become the new host of the Tal symbiont. In 3187, two years prior to the events of Star Trek: Discovery season 3, Adira and Gray were aboard a Generation Ship traveling to the United Federation of Planets' headquarters' new location. When their ship was attacked, Gray was mortally wounded; to protect the Tal symbiont, it was placed inside Adira, although she is human. And because Adira is not Trill, she was unable to access the vast majority of the previous Tal hosts' memories, including Admiral Senna Tal, and Adira had also forgotten about Gray. In episode 3, "People of Earth", Adira joined the crew of the U.S.S. Discovery and she revealed her dilemma to Burnham and Captain Saru (Doug Jones), who need the memories of Admiral Tal so they could also locate the Federation's new headquarters.
In "Forget Me Not", Michael and Adira were initially welcomed by the Trill, who were overjoyed to learn that a host of a symbiont had returned to their homeworld. But they rejected Adira as the Tal symbiont's host because she is human. Still, Burnham and Adira found Trill Leader Pav (Andreas Aspergis) sympathetic. Pav led them into the sacred pools in the Caves of Mak'ala so that Adira could access the Tal symbiont - and Michael also entered the pool when Adira vanished in the waters. Ultimately, Adira's experience was of the Tal symbiont finally accepting her and she was able to meet all of the previous Tal hosts, including Senna and Gray, and gain access to their memories. When Adira emerged, the Trill also acknowledged the human host as Adira Tal and allowed her to return to the Discovery with Michael. And thanks to Senna Tal's unlocked memories, Adira was able to provide Michael with the coordinates to Federation headquarters.
The final surprise of "Forget Me Not" was that Gray manifested independently of Adira's memories and became a vision that, presumably, only Adira can see, talk to, and interact with. For fans of Battlestar Galactica, Grey's presence was remarkably similar to how the Cylon Number Six (Tricia Helfer), known as Head Six, could 'haunt' Dr. Gaius Baltar (James Callis). But Star Trek: Deep Space Nine did this same narrative trick years before in the season 7 episode, "Field of Fire", where Ezri Dax interacted with a prior Dax host, Joran Dax (Leigh J. McCloskey).
Joran Dax, who lived in the 23rd century, was an outcast host whose memories were buried by the other Dax hosts, including Ezri's predecessor Jadzia (Terry Farrell), because Joran was a murderer. However, it's exactly Joran's unrepentant qualities that a reluctant Ezri needed to advise her in order to catch a killer aboard Deep Space Nine in "Field of Fire". But to access Joran and manifest his presence, Ezri performed the Trill Rite of Emergence.
However, both Adira and Gray were perplexed at how the former Tal host was able to appear before the current Tal host, and, probably because they are both so young, neither seemed to be aware of the Trill Rite of Emergence. Unless somehow, the Rite was performed while Adira was in the pool on Trill, which viewers didn't witness, it's a mystery how Gray and Adira are able to interact the way Ezri and Joran did on DS9 800 years prior. But Star Trek: Discovery's 32nd century is filled with numerous changes and conundrums, and the open questions left by Adira, Gray, and the Trill are now the latest the series must solve.