Star Trek: 10 Crazy Facts About The Trill
5. The Guardians
For many, if not most, Trills, the ultimate goal was to become joined. Since the Symbiosis Commission strictly controls the number of candidates selected for the honor, not everyone had the opportunity. For some Trills, there was another calling that puts them in the service of the symbionts.
The un-joined symbionts that remain in the Caves of Mak'ala to propagate the series still require care to stay alive. They require very specific conditions to survive outside of a Trill host. That's where the Guardians come in.
Comparable to a monk who has taken a vow of celibacy, the Guardians have chosen to not be joined and to care for the un-joined symbionts in the Caves. They're not necessarily good with people, though, since they spend all their time in caves with sentient slugs swimming in milky pools of liquid.
The Guardians also served the important purpose of facilitating the Zhian'tara ceremony by transferring the previous host's personalities from the symbiont into the guest hosts.