Kotaro Inugami is initially introduced as one of the three mercenary children aiding Chigusa in Kyoto. He is a dog hanyō, evident from his ears and tail. Despite his impulsive nature, Kotaro is chivalrous and refuses to hit women unless they are formidable fighters. He trained under the Garyū Inugami-ryū, essentially a self-taught martial arts school. Kotaro harbors a strong dislike for Western-style magic, though Negi is one of the few exceptions. An expert in ninjutsu and charms, he can summon inugami (dog spirits) but prefers close combat over using magic from afar. Kotaro also possesses the ability to fly using an adaptation of his inugami powers. Additionally, he can transform into a small black and white dog with a paper charm or into a stronger beast form. He claims that most of his fighting techniques, including ki power usage, were developed by himself. His accurate analysis of Negi's techniques supports his claim.
After the Kyoto Arc, Kotaro was placed under the custody of the Kansai Magic Association, where his transformation and spirit-summoning abilities were sealed. He later escaped to find Negi, whom he now considers a rival due to being the only person his age capable of fighting him on even terms. Upon arriving at Mahora, he encountered Graf Hermann and The Slime Sisters, learning about their plan to attack Negi. Kotaro fought them, stole the bottle that released Graf Hermann, and escaped. However, while trying to warn Negi, he was injured and hit with a confusion spell, resulting in temporary amnesia. Chizuru and Natsumi, initially mistaking him for a stray dog, nursed him back to health.
Kotaro has had a very lonely past, being an outcast from both human and dog-yōkai communities due to his hanyō status. This makes him cherish his friendship with Negi and his life with Chizuru and Natsumi, who become his first true friends and family. Chizuru’s maternal care often embarrasses him, especially her habit of hugging him in public. He frequently scolds Negi for being involved with so many girls, only to have his relationship with Chizuru thrown back at him, leaving it unclear whether this is due to a crush or simple embarrassment. Kotaro likes to think of himself as a "lone wolf," but Negi teases him by saying, "You're more like a dog." His views often lead to arguments with Yue, which he usually loses. He holds Kaede in high regard because of her strength.
Like Negi, Kotaro has a somewhat limited worldview and is largely ignorant about the emotions of girls. His focus on becoming the strongest only in combat leaves him unprepared for situations that do not involve fighting.
