Security chief aboard
Deep Space 9 during the
Cardassian ownership of the
station, as well as later when the
Starfleet took over the
facility in 2369.
Odo was a
shape-shifter, who was
found in the
Denorios Belt, but it was not
known where his species
originated, nor had he ever encountered another
being of his
kind.
Odo maintained a humanoid form while at work on the station, however, he had to return to his natural form, a viscous orange fluid, every 16 hours. He didn't need to eat and only had an approximation of a mouth and digestive system and had no sense of smell. He patterned his own hairstyle after the scientist assigned to study him at the research center on planet Bajor. Being the only one of his kind, he attempted to fit into society by being the 'life of the party'. He'd turn himself into any object requested by the partygoers, which only increased his feelings of isolation and loneliness. He did not even know how to gamble. Odo never took a mate, claiming that he would not want to make the compromises that a relationship would demand. Odo has also had an uncompromising view of law enforcement, believing that "laws change, but justice is justice."
Odo nevertheless yearned to learn of his origins, and hoped one day to meet another individual of his species. When Crodin of the planet Rakhar gave him a shape-shifting necklace, Odo allowed himself to hope that he might find other shape-shifters in the Gamma Quadrant. Odo did let his guard down slightly with Ambassador Lwaxana Troi in 2369, when the two were trapped in a turbolift in the station. In their different ways, both Odo and Troi were loners, and their enforced proximity caused them to share each other's vulnerabilities.
Odo eventually found his home in 2373, when the Founders made their presence known to the Alpha Quadrant. The Founders, Odo's race, informed Odo that he was one of the 'hundred' sent out to explore the galaxy for the Founders, and that they hadn't expected to hear from him for another 175 years.
The Great Link, the term for the collective consciousness of the Founders, was successful at luring Odo into the Gamma Quadrant, along with his Deep Space Nine crewmates.
Odo was aboard the Federation starship Defiant, when a shape-shifter, disguised as Julian Bashir, infiltrated Deep Space Nine, and eventually the Defiant. Defending the ship and his friends, Odo confronted the rogue changeling, and eventually killed it. The next year, with Odo in the hands of the Founders, they delivered his sentence for killing a fellow changeling, the first time another changeling has harmed another. They reasoned, since he wanted to live with the solids, he could become a solid.
With Odo a solid now, and his abilities vanished, he found himself asking himself whether living was worth it, and eventually fell in love in Major Kira Nerys.
After Benjamin Sisko's's death at the hands of the Pah Wraiths, Odo found himself disenchanted with his life away from his home, and rejoined the Great Link. This was only after the war with the Dominion and Alpha Quadrant was ended with the treaty.
Odo is currently with the Great Link.