Odan was a Trill symbiont introduced in Star Trek: The Next Generation episode The Host.
Odan was an old symbiont even in the 21st century, having been joined eight times by the mid 21st century. As Darzan Odan he served as Lela Dax's advocate when she faced treason charges for trying to communicate with the L'Dira.
In the 23rd century Odan was joined to a Trill male named Norvo. As Norvo Odan he was the Ambassador to the United Federation of Planets when Trill was applying for membership in the Federation. He recommended to his superiors that the Trill keep the existence of the symbiont a secret, fearing that the people of the Federation would not understand the symbionts, and feared they would wish to harm the symbionts.
In the 24th century Odan was hosted by two male Trills, both of whom served as diplomats. When the first of the two hosts died, Odan was moved to a new host, who presented the previous Odan as his father. In 2367 he was on board the Enterprise-D on a mission to help the two moons of Peliar Zel from going to war with each other. During this time Odan entered into a relationship with Doctor Beverly Crusher. The secret of the Trill symbionts came out into the open when Odan's ship was attacked during negotiations. The host was fatally injured. Before the host died he revealed the existence of the symbiont to Dr. Crusher and begged her to save the symbiont's life.
To keep the symbiont alive long enough for a new host to arrive William Riker hosted Odan. In Riker's body Odan was able to convince the ambassadors that he was Odan, and helped them come to a peace agreement. However the hosting was hard on Riker, whose body rejected Odan as a foreign invader. Odan ordered that the Enterprise remove him after the peace talks were concluded, even if the new host had not arrived, in order to keep Riker from being permanently damaged.
After the talks were concluded Odan was removed from Riker's body. Odan's new host Kareel arrived a short time later and Odan was implanted inside her. The joining was successful and Kareel Odan recovered a short time later. Dr. Crusher decided to end the relationship as the changes were too much to keep up with.
The host Kareel was killed by the Trill terrorist group led by Verad Kalon in 2376. Odan temporarily was joined with Beverly Crusher to work on a cure to a disease engineered by Kalon to target the symbionts. Afterwards Odan went on to a new Trill host named Kinjer Odan.
