Jaken

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

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

History
In the early part of the story, Jaken uses the Staff of Two Heads to find Inuyasha's and Sesshōmaru'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 Sesshōmaru to save her. He also worries if Rin gets hurt that Sesshōmaru 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 Sesshōmaru'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 Sesshōmaru and always praises Sesshōmaru whenever he defeats a demon, much to Sesshōmaru's annoyance. Unlike Myoga, Jaken always sticks by Sesshōmaru's side even when times of danger.
 * Sesshōmaru

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 Sesshōmaru. Inuyasha also seems to bully him. As when Jaken started to scold him about Inuyasha calling Jaken "small", the hanyō grabbed him and tried to stretch him out until Kagome told him to "Sit".
 * Inuyasha

Manga vs. Anime
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 Tessaiga). During battle with another yōkai tribe, the enemy leader had Jaken by his neck when Sesshōmaru 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 Sesshōmaru, who allowed him to become a servant if he could use the Staff of Two Heads.