Jaken

Jaken (邪見) is a small green imp-like yōkai who is extremely loyal to Sesshomaru, often praising his master's greatness although Sesshomaru usually ignores him. Jaken himself is not especially powerful, despite bearing a weapon known as the Staff of Two Heads (人頭杖, nintōjō). He seldom engages in battle and is actually killed by Kaijinbo with Tōkijin, but Sesshomaru resurrects him with Tenseiga.

biography
The manga explains little about Jaken's past, but the anime shows that Jaken was once a lord among similar demons. During battle with another yōkai tribe, the enemy leader had Jaken by his neck when Sesshomaru appeared and told the demon to move out of his path. When this was not obeyed, he destroyed the yōkai with this poison whip. Out of a sense of obligation, Jaken followed Sesshomaru, who allowed him to become a servant if he could use the Staff of Two Heads.