A Trill guardian who helped Adira unlock the memories of the Tal symbiont and later tested the seekers of the Progenitors' technology.
Xi is a Trill guardian who appears in Forget Me Not. When Michael Burnham brings Adira to the Trill homeworld, many Trill refuse to accept that a non-Trill could host a symbiont, and a group of them tries to kill Adira and take Tal. After Burnham stops them, Guardian Xi offers to help and guides Adira at the Caves of Mak'ala to connect with the memories of previous hosts.
Xi returns in Jinaal, posing a riddle to Burnham before the crew is allowed to consult the memories of the scientist Jinaal.