Jaken is a kappa demon who serves as the loyal, if often beleaguered, assistant and retainer to Sesshomaru. His name means wicked perspective in Japanese. Before becoming Sesshomaru's servant, Jaken was a minor lord among similar, weaker demons. His life was saved when Sesshomaru effortlessly destroyed a more powerful demon who had Jaken by the throat, simply because the creature was blocking his path. Captivated by Sesshomaru's immense power and beauty, and feeling a deep debt of gratitude, Jaken abandoned his own tribe to follow him. To prove his worth, Jaken was tasked with learning to use a powerful artifact, the Staff of Heads, and upon succeeding, was officially accepted as Sesshomaru's servant.

Personality-wise, Jaken is a complex figure of contradictions. He is exceedingly arrogant, pompous, and quick to boast about his own importance or the greatness of his master, often looking down on humans and other demons he deems inferior. Simultaneously, he is physically weak, cowardly, and aware of his own limitations, which makes him a frequent source of comic relief. He often mutters about how his mother was right and he should have become a ferryman, lamenting his difficult life. Despite his bluster and constant self-pity, Jaken's defining trait is his unwavering loyalty to Sesshomaru. He is deeply devoted and seeks his master's approval in all things, faithfully carrying out errands and acting as a herald, bodyguard, and caretaker.

Jaken's primary motivation is to serve Sesshomaru and prove his own usefulness. He frequently concocts elaborate, and often comically failed, schemes to please his master, such as attempting to steal Inuyasha's powerful sword, Tessaiga. His role in the story is multifaceted. He acts as Sesshomaru's constant companion and mouthpiece, explaining his master's thoughts and motivations to others while also providing a contrasting perspective through his own fear and worry. On a narrative level, he is essential comic relief, his mishaps and exaggerated reactions softening Sesshomaru's cold and stoic presence. He also often serves as a caretaker and reluctant friend to Rin, the human girl who travels with them.

Jaken's key relationships define his existence. His relationship with Sesshomaru is one of absolute fealty. Sesshomaru saved his life and accepted him as a servant, and in return, Jaken offers total devotion. Sesshomaru, for his part, shows a level of care for his vassal, resurrecting him with the Tenseiga after he is killed by the demon Kaijinbo and protecting him from powerful enemies. His relationship with Rin evolves significantly. Initially disdainful of her because she is a human child, Jaken eventually develops a grudging affection for her, acting as her guardian and becoming concerned for her safety. This bond is highlighted when he is terrified after Rin is kidnapped, knowing his master will be enraged, but also because he genuinely cares for her. In contrast, his relationship with Inuyasha is one of bitter rivalry and conflict, fueled by his loyalty to Sesshomaru, even though Inuyasha has inadvertently saved his life on multiple occasions.

In terms of development, Jaken does not undergo a dramatic transformation, but his role solidifies from a simple sycophantic servant into a more reliable and integral member of Sesshomaru's small group. His bond with Rin deepens, and he becomes less of a pure coward and more of a desperate but dedicated underling who will take initiative, however poorly, to serve his master's presumed interests.

Jaken possesses few remarkable innate demonic abilities. He has heightened senses, can project a weak demonic aura, and has a greater resistance to cold than a human. Like many demons, he ages extremely slowly, being at least 150 years old at the start of the story. His true utility and notable abilities come from his use of the Staff of Heads, a weapon given to him by Sesshomaru. This staff has two heads, one of a woman and one of an old man. It can sense and locate other powerful items or beings, such as the tomb of Sesshomaru's father. More offensively, the old man's head can spew powerful flames capable of destroying many enemies at once, though it is most effective against weaker opponents. On one occasion, he also used the staff to conjure a hot spring that released paralyzing fumes. While he uses this weapon, his own physical strength and demonic power remain quite weak, and he is easily overpowered.