Odo is the security chief on Deep Space Nine station (the character has come to be known colloquially as "Jello Man"). He was security chief for the Bajorans before the Federation came to DS9 and was the Cardassian security chief on DS9 before that.

He is a shape-shifter (also known as a Changeling or Founder of Dominion), much like Martia in ST6. He was found alone 50 years ago on a mysterious derelict spacecraft that appeared in the Denorias asteroid belt and is from an unknown race. He was found by the Bajorans and lived amongst them. And he was raised by the Bajoran scientist, Dr. Mora ("The Begotten").

At first he was sort of an Elephant Man, a source of curiosity and humor as he turned himself into a chair or pencil. Finally he realized he would have to take the form of a humanoid to assimilate and function in their environment. He does it, but resents it. As a result, Odo performs a uniquely important role in the ensemble: he is a character who explores and comments on human values.

Because he is forced to pass as one of us, his point of view usually comes with a cynical and critical edge. But he can't quite get it right, this humanoid shape, though he continues to try. So he looks a little unfinished in a way. He's been working on it a long time. Someone might ask him: Why don't you take the form of a younger man? His answer: I would if I could. In an alternate timeline, a 200-year-older Odo had a much better humanoid facial resemblance. (stardate 50814 "Children of Time") He has the adopted child syndrome, searching for his own personal identity. He became a law man because he is certain that justice is an integral part of their being, since the necessity for it runs through every fiber of his body -- a racial memory.

He has a couple of Bajoran deputies; he doesn't allow weapons on the Promenade, and once every day he must return to his gelatinous form. He has no sense of smell. In his early life, he was hoping someday that a ship could come through the wormhole whose crew could tell him who and what he was.

Odo found his homeworld which is in the Omarion nebula on stardate 48212 (in year 2371) ("The Search, Part I & II""). On his homeworld, there are many other shape-shifters, Founders of the Dominion. Later in the same year, Odo killed a fellow changeling, who attempted shifting into Ambassador Krajensky to cause wars, on stardate 48959 ("The Adversary"). One year later, Odo was punished by Founders (the Great Link) for being the first Founder to kill a changeling. They made Odo to become a solid (no shape-shifting ability) on stardate 49962 ("Broken Link"). Later on stardate ~50450 (in year 2373), due to a baby Changeling, Odo regained his shape-shifting ability ("The Begotten").

Odo was in love with Kira, but Kira didn't know until an alternate Odo told her on stardate 50814 (in year 2373) ("Children of Time").

In order to keep the writers from using his shape-shifting ability to constantly save the day, we find that his ability to assume ANY form is very taxing and he must rest at the end of each day in his natural form, rather like a bowl of Jello.

He was there when the Feds took over the station from the Cardassians, and has his own way of doing things, though he wants to become more human (all Star Trek shows have to have ONE character who wants to be human). He'll take the law into his own hands to make things the way he wants.

He has been on the station for at least four years prior to the Federation takeover in "Emissary. (also in "Things Past") It is interesting that Odo doesn't know about others like himself when Martia (Star Trek 6) is from a known race called the "Chameloids" and we had the Alassomorph shapeshifters from "The Dauphin."