Myoga is a flea and he is also Inu Yasha's vassal. He served under Inu Yasha's father, but when he passed on his services transfered to Inu Yasha. Myoga is wise, but often cowardly, and you know trouble is coming, if he is seen running away. He often makes his entrance by sucking on Inu Yasha's neck.
Myoga often explains things to Inu Yasha about his father, or his demonic self. Myoga is very wise and has been around long enough to know almost everything about Inu Yasha and his family.