Variously known as Shape shifters1, Changelings2 or Founders3
, Odo's people are one of the most intriguing life forms ever encountered by the Federation. Their natural state is that of a viscous liquid which has no apparent internal structure, no organs or other biological systems. Nevertheless it is capable of a massive degree of flexibility; by a simple act of will Odo is able to alter his shape, surface texture, volume4, and even his mass.2 The alterations of shape and texture have been observed in several other shape changing species such as the Chameloids.5
Alteration of mass is rather more difficult and how this apparently impossible feat is accomplished remains something of a mystery. Some have suggested that the Changelings are multi-dimensional beings who exist mostly outside our own space-time. According to this theory the being we see as a Changeling would comprise only a small portion of the whole; by moving more of less of itself into this universe the Changeling would be able to alter its apparent volume and mass. In theory this would allow them to withdraw themselves from our universe completely, giving them effective invisibility and the ability to pass through walls or other barriers. In fact, Changelings have never demonstrated this ability and so if the theory is true there must be some inherent limitation on the degree of this multidimensional ability.
In 2371 Odo accompanied the USS Defiant on a mission to the Gamma Quadrant. Whilst there he found himself irresistibly drawn to the Omarian Nebula, where he discovered the home world of his people.6 It transpired that the Changelings were the infamous Founders of the Dominion, which they had created in order to insulate themselves from the prejudice they had encountered from those they call "The Solids". Odo refused to join the Founders7, although during the Dominions occupation of Deep Space Nine he did waver in his loyalties considerably.8
Odo has long held a romantic attraction for Colonel Kira, but for many years he kept this a secret because he feared rejection.9 In 2373 his feelings were finally revealed during the Defiant's encounter with an alternate timeline version of Odo10, but he still made no attempt to pursue a relationship until the following year when the hologram Vic Fontaine pushed the pair into confronting their true feelings. Odo and Kira finally became involved11, and remained a couple until the end of 2375.12
In 2375 Odo began to display signs of a serious illness. Doctor Bashir discovered that he had been deliberately infected with a terminal disease by Section 3113; during his time in the Great Link Odo had passed the disease on to the rest of the Founders, infecting the entire species.12 Bashir was able to develop a cure for the disease14, and in the final stages of the war Odo passed the cure onto the female Founder in order to convince her that solids could be trusted. She agreed to stand trial for her crimes, and in return Odo agreed to go back through the wormhole and cure the rest of the Great Link. This marked the end of Odo's relationship with Colonel Kira.12