The Trill woman sent to become the new permanent host of the symbiont Odan after Riker carried it temporarily, in the fourth-season episode The Host.
Kareel is a Trill host. She appears at the end of the fourth-season episode The Host, the story that introduces the Trill as a joined species.
Ambassador Odan, with whom Beverly Crusher has fallen in love, is fatally injured on the way to mediate between the two moons of Peliar Zel, and Crusher learns that the ambassador is in fact a symbiont living inside a host body. Riker volunteers to carry Odan until a replacement arrives from the Trill homeworld, nearly at the cost of his life.
The replacement is Kareel, a young woman. Once the transfer is complete, Odan, speaking through her, still loves Beverly. Crusher admits she cannot keep up with such changes, and they part affectionately.