Erizmus Taggart is Commander Owain Taggart's symbiont. He was an old man living on an asteroid, after having brushed with death and escaped during the war of what has come to be known as the integration of two species, now simply known as Trill. Most Trill these days sport spots, which is actually a newer subspecies actually created due to the war between an invading force from a species off-world. Erizmus is of the older variety of Trill, with no spots, and instead sports a V shaped bump on his head (see TNG for its version of the Trill and Memory Alpha info on Trills for the non-canon explanation between the differences between Trills. This backstory is a mixture of both.)
When Owain was scheduled to be joined with Erizmus, the old man in his last hours, Owain was reluctant to join. In fact, he disbelieved in the process of joining and felt it was a burden on Trills and their character. Because of the contraversial nature of the passing of memories from one body to the other and the effect of two different personalities, Owain felt it wasn't right. But he had been forced by his mother, and had been mad at her for it for some time, until he realized that Erizmus was actually a blessing to him.
Erizmus now appears visually from time to time in the Holodeck where Owain has developed a way to interact with his Symbiont instead of awkwardly speaking to him through his mind.