Odo was a Changeling individual. Odo a member of the Hundred, one hundred young and unformed Changelings, who were sent out by the Founders of the Dominion. Discovered by the Cardassians near Bajor and classified as 'Odo'ital' (unknown sample), Odo was studied by Mora Pol who was successful in getting Odo to assume a humanoid form.
Odo later became an impartial law enforcement figure on Terok Nor and joined the Bajoran Militia when the Occupation of Bajor ended, remaining on the station now named Deep Space 9 by Starfleet. Odo would later learn that his people were the leaders of the Dominion, a fact he was never comfortable with. After the Dominion War, Odo returned to the Great Link.