Star Trek Expanded Universe

Drel was a Trill symbiont. In the early 25th century was among the youngest symbionts in Starfleet, having been joined to only two hosts. Nevertheless, the symbiont had over a century of memories when it was joined to its third host in 2407. (Star Trek Online: Valkyries)

Drel was first joined in 2295 to host Perciv and almost immediately left the Trill homeworld to join Starfleet, a decision many older Trill, including Quinn, considered a rash attempt to take on too many experiences at once. This started a longstanding conflict between Drel, the Symbiosis Commission, and Quinn. The conflict carried over to Drel's second host, Antori, though the Commission was willing to give him a chance by granting initiate Carlin Agran's request to have Antori Drel as her field docent in 2405. ("The Best Revenge")

Drel made a point of requesting a specific host each time it was joined, each time choosing individuals who had "spunk." By 2407 Antori Drel had made provision in his will that Carlin become the next host of the Drel symbiont in the event of his death. When he was killed on Yamatai later that year, Drel was joined to the Carlin host in order to save its life. ("Survivors of Yamatai")