Origin

Odan is a joined Trill and is a legendary ambassador who negotiated a peace treaty between Alpha Moon and Beta moon.  Thirty years later when hostilities were rejoined Odan was called back to mediate.  Odan is the name of the symbiont and the original Trill host was dead so Odan came back and claimed to be the son of the original Odan.
 
Odan fell deeply in love with Beverly Crusher on the U.S.S. Enterprise D.  The relationship was very passionate and also tragic when Odan's host was killed.  The symbiont almost died as well but William Riker was able to join with the host temporarily and keep it alive Odan also tried to keep the romance going with Riker's host body.  He also tried to keep it going when his host became a female when Kareel came to the Enterprise to be joined with the Odan host becoming Kareel Odan.  Beverly could not go to that point and they broke their relationship.

Creation

Odan was created for a Star Trek: The Next Generation episode.  Kareel just comes in at the end of the episode but later appears in comics.

Character evolution

Kareel was killed on a ship by Trill terrorists who wanted to destroyed all Joined Trill.  The U.S.S. Enterprise E this times helps the symbiont goes back to the home world to save it.