Trill

The Trill are a humanoid species. A small minority, after a rigorous selection process, is permitted to join with a sentient, intelligent symbiont. The symbiont is long-lived and can pass from host to host, carrying all the memories, skills, and experiences of each prior host. Trill symbionts are also capable of joining with human hosts.
The Trill made their debut on television in the Star Trek: The Next Generation episode "The Host" (May 11, 1991), and were further developed in Star Trek: Deep Space Nine. The Trill Jadzia Dax is the 8th host of the symbiont Dax, and together they are one of the main characters of Deep Space Nine for the first six seasons; when Jadzia is killed, Ezri Dax becomes the next Dax host for the seventh and final season. This species was also briefly represented as a holonovel character corresponding to Ensign Harry Kim on Star Trek: Voyager in "Author, Author" (April 18, 2001).
Adira Tal, a human joined with a Trill symbiont, appears in the third season of Star Trek: Discovery.[28] A Trill doctor, Naáshala Kunamadéstifee, appears in Star Trek: Picard,[29] and several Trill also feature in Lower Decks.[30][31]
Trill has been studied in the analysis of the biology of Star Trek, especially in regards to the symbionts.[32]
There are two contrasting concepts for Trill. One is that the symbiont is essentially an alien person;[32][33] nonetheless, the joined Trill still mixes the original person with the memories and some of the personality of the symbiont.[33] Only a small percentage of Trill are joined, and being accepted for the process is considered an honor.[33] For joined Trill, a symbiont's memories, and to some extent personality, are synthesized with the existing Trill's personality.[33] Joined Trill have been studied in the philosophies of Star Trek, in particular whether a person is essentially the sum of their memories (the philosopher Locke's "memory theory").[34] This concept was explored in the Star Trek:Deep Space Nine television episode "Dax".[34] (see Personal identity § Locke's conception)
The contrasting philosophy of the symbiont is called "functionalism" according to Star Trek and Philosophy: The Wrath of Kant, in which people are defined by their actions as opposed to memories.[35] The symbionts have been dismissed as "just memories"[36] rather than a true person, although in other cases they are described as a "sentient symbiotic organism".[33]