He was not to be a joined Trill, but events caused Rico to be joined with the Syimbiont Barak. Together they are a good pair, but Rico has had a hard time getting use to some of the things that Barak likes.

Barak had caused the two to be arrested for abandoning Starfleet, because the Trill Homeworld was under attack from the Orion Syndicate. They went to fight for their homeworld.

They served their time and was released, because of the need for expierenced officers. Barak has been alive for well over a hundred years and has many hosts. One being married to an Andorian Male Ambassador at one time.

Barak was a special case symbiont. When he was born he was going to die. His first host, the female Trill, that had married the Andorian was born with a special medical condition. At a very great risk, they were joined. They both lived, and the host had lived for well over a hundred years. The early joining had given both the host and symbiont the immuninty they needed to live.