Overview
One of "the hundred" sent out by The Founders long ago to explore the galaxy, Odo was found adrift in 2356 in the Denorios Belt by Bajorans. It is unclear how long he had been adrift. Since Odo had not yet learned how to morph into a humanoid appearance, he appeared in his natural gelatinous form. Bajoran scientists, not sure of what Odo actually was, put him in a container and labeled it "unknown sample". The Cardassian overseers translated this into their own language as "odo'ital", which literally means "nothing". Even after it became clear that Odo was sentient, the scientists kept calling him that, giving him the name 'Odo Ital' (like a typical Bajoran name), which was eventually shortened to simply "Odo" (Episode "Heart of Stone").
He was studied by Bajoran scientist Dr. Mora Pol, whom Odo disliked due to Mora's questionable (but effective) practice of inducing pain as a motivator in encouraging Odo to assume different shapes before, and even after, Odo was recognized as sentient. While Mora regarded himself as a father of sorts to Odo, Odo felt intense hostility toward Mora. Their relationship was difficult, but eventually they did reach a certain understanding. Odo modeled his humanoid appearance on Dr. Mora, but never had the skill to duplicate humanoid features with precision, making his humanoid appearance an approximation.
As a Changeling, Odo can alter his appearance at will to camouflage his presence. He can change the shape of body parts as needed, such as lengthening an arm into a tentacle to grab a fleeing individual, and is able to liquefy himself in order to flow into otherwise inaccessible areas. After 16 hours in solid form, he must revert to his gelatinous state in order to rest. Originally he uses a bucket for this purpose, which he keeps in his office, but he gives it up after getting his own quarters in the Season 3 episode "The Abandoned." From this point on, he uses the bucket as a pot for a house plant given to him by Kira Nerys.
When others of Odo's species (the Founders) are later introduced, they share a similar "unfinished" look, which they adopt to make Odo feel at ease around them as they are able to replicate humanoids exactly, whereas Odo, having grown up without others of his kind, never learned to master it. The other Founders refer to non-shapeshifters as "solids" or "monoforms", while Odo occasionally calls them "you humanoids".
Due to his status as an impartial outsider, he was sometimes employed by Bajorans to settle disputes. Odo later worked for the Cardassians during the Occupation of Bajor aboard the orbiting Terok Nor ore-processing space station, and later for the Bajorans/Federation aboard Deep Space Nine (the Federation name for Terok Nor), in both cases as chief of security. During the occupation, Gul Dukat had originally enlisted Odo to investigate the murder of one of the Bajoran station workers. Since Odo was seen as a neutral observer, sympathizing with no one, he was considered a valuable security officer.
Odo's early career on Terok Nor hit a snag when he investigated an assassination attempt on Dukat. Three Bajoran workers were initially linked to the attack, and Odo, eager to enforce the rule of law and quickly conclude the investigation, hurriedly convicted them and allowed them to be executed. Soon after, Odo discovered that all three men were innocent and that his lax investigative procedures caused a serious miscarriage of justice. This prompted Odo to be much more insistent on actual justice, though not necessarily the actual letter of the law.
Up until this time, Odo was unaware of his people and his ancestry. A criminal showed him a necklace containing an apparently non-sentient shape-shifter in the episode "Vortex". Later, in the episode "The Alternate", Dr. Mora visits Odo at Deep Space Nine and travels with him to the Gamma Quadrant to investigate a planet with a similar life-form. It isn't until later that Odo learns that his people are the leaders of the Dominion.
In 2372, Odo marries a pregnant, recently divorced Lwaxana Troi as a favor to her, so that her child will not be taken away from her at birth by the father, as required by Tavnian law. Although Odo had been annoyed by Lwaxana in the past, he has come to appreciate her company. However, he still is not in love with her, so after her ex-husband witnesses the marriage, she returns to Betazed.
Odo becomes the first ever Changeling to kill another of his species (in "The Adversary"). Although it was in self-defense, and there was no intent to kill, the Founders punish him by taking away his shape-shifting abilities, then transforming him into a physical human being ("Broken Link"). He retains his physical body for several months and even has a brief romantic relationship, but is unhappy in this form. Eventually, an infant shape-shifter Odo cares for dies, but before doing so integrates its gelatinous form into Odo's physical makeup. This causes Odo's body to revert back to normal, and he regains all of his shape-shifting abilities ("The Begotten").
Odo remains loyal to the Bajoran and Federation people throughout the Dominion War. At the end of the war, he goes back to the "Great Link" to cure the other Founders of a disease that Section 31 had infected them with and to teach the Founders how to trust other races again.
During his time on the station, Odo is involved in an ongoing battle of wits with the Ferengi, Quark. Odo often spends a great deal of time observing Quark, in humanoid form as well as various other shapes. Over the years the two men would gain a grudging respect for each other. In the series' final episode, "What You Leave Behind", Odo refuses to admit the friendship he had developed with Quark, turning away with his usual contempt. But Quark sees through this, saying, "That man loves me. Did you see it? It was written all over his back."
Odo enters a romantic relationship with Kira Nerys with the help of Vic Fontaine. Odo had pined for her for years, while she was oblivious. They remain involved even after Odo leaves to rejoin the Great Link. He returns to the station for a while in the novel "Unity", first under disguise as a Trelian woman named Wex. He later reveals himself to Kira during a conflict at one of the temples on Bajor. The two pick up where they left off even though Odo has to leave again, at least for a while.
In the Voyager episode, "Non Sequitur," Tom Paris' alternate self (in the parallel dimension) relates to Harry Kim a story about how, during a stop at Deep Space Nine, he got in a bar fight with a Ferengi and was thrown in the brig by "a very unpleasant shapeshifter", obviously referring to Odo.
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