It is a mysterious creature that should probably be better studied as its habitat and mating and such is not well known. It is also a declining species of fish.
I will say this though I am a huge fan of Unagi. It tastes great. I wouldn't really know how to describe it as it is a fish unlike all other. It has a fluffy component to it when cooked correctly. The sauce put on the unagi is usually a sweeter sauce. Condiment often also associated with unagi is sanshou (Sichuan Pepper) which gives a slight numbing sensation to the tongue.