In human form: a pilchard is a no good, manipulating git who slimes into your life without warning, often in an underhand and devious manner. The pilchard, or pilch, (if you will) will take you for all you are worth, sucking you dry of emotion, money and soul. They have a distinctive look (like Eddie Windass off Coronation Street) but this may be overlooked due to their uncanny ability to fit in and mingle with the everyday folk and ultimately defecate on the lives of all they come into contact with.
A pilchard is actually a fish: Any of various small marine fishes related to the herrings, especially a commercially important edible species, Sardina pilchardus, of European waters.
The distinction here is that a pilchard is quite a stupid, inconsequential fish, but it is edible, so it has redeeming qualities.