Jaken

is a kappa demon who is extremely loyal to Sesshomaru, often praising his master's greatness.

Although Jaken is not as powerful as his employeer, he is skilled at wielding the Nintoujou (人頭杖, Staff of Two Heads). He seldom engages in battle and is actually killed by Kaijinbo with Tōkijin, but Sesshomaru resurrects him with Tenseiga.

Biography
In the early part of the story, Jaken uses the Staff of Two Heads to find Inuyasha's and Sesshomaru's father's grave. He eventually became not much more than comic relief, having hardly any role in the story.

Relationships
At first Jaken disliked Rin as she was a human. However, as time went by he began to treat her better. Jaken sometime acts as if he hates her, but in truth he doesn't want Rin to get hurt and whenever there's trouble, Jaken always tells Sesshomaru to save her. He also worries if Rin gets hurt that Sesshomaru will kill him. Rin often likes to make up songs such as "Jaken-sama, Jaken-sama, why are you green?" When Rin dies, Jaken is shown crying. Whether they are just for Sesshomaru's sake, we don't know, but before she died, he was very sad when he found out she was going to.
 * Rin

Jaken respects Sesshomaru and always praises Sesshomaru whenever he defeats a demon, much to Sesshomaru's annoyance. Unlike Myoga, Jaken always sticks by Sesshomaru's side even when times of danger.
 * Sesshomaru

Jaken and Inuyasha have a very bitter relationship. Despite the number of times Inuyasha has saved him, he always turns against him due to the conflict between Inuyasha and Sesshomaru. Inuyasha also seems to bully him. As when Jaken started to scold him about Inuyasha calling Jaken "small", the hanyou grabbed him and tried to stretch him out until Kagome told him to "Sit".
 * Inuyasha

Anime/Manga Difference
The manga explains little about Jaken's past, but the anime shows that Jaken was once a lord among similar demons (episode 79 "Jaken's Plan to Steal Tetsusaiga). 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 calmly destroyed the Yōkai with his 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.