Myouga is a flea demon.  He used to be the servant of Inuyasha's father.  He then supposed to be the guard of Inuyasha's father's grave.  But he ran to Inuyasha after Sesshoumaru came looking for the grave.  He's a real coward and runs away every time there's danger (I guess it's better to be a live coward than dead idiot).  One of his specialties is that he can suck out poison.  He helped out Inuyasha by giving advices and information.