Kotaro Inugami is first seen as one of the three mercenary children helping Chigusa in Kyoto. He is a dog hanyō, which is evidenced by his ears and tail. Despite being hotheaded, Kotaro is a chivalrous person and would never hit a woman unless she is a powerful fighter, having trained under the Garyū Inugami-ryū which is basically a self-taught school. He also has a strong dislike of Western-style magic, with Negi being one of the few exceptions. Kotaro is an expert in ninjutsu and charms, and has the power to summon inugami (dog spirits), but prefers to fight hand to hand rather than using magic from a distance. Kotaro is also capable of self propelled flight with an adaption of his inugami abilities. Kotaro also has the ability to transform into a small black and white dog using a paper charm, or to a stronger beast form. He claim that most of his fighting technique (including the use of ki power) was made up by himself. His capability to correctly analyze Negi's technique supports his ability to achieve such a task.

After the Kyoto Arc, he was put under the custody of the Kansai Magic Association, where his abilities to transform and summon dog spirits were sealed. He later escaped to find Negi (whom he now considers as a rival, being the only person he's met of his own age who can fight with him on even terms) to have another rematch. When he arrived at Mahora, he ran into Graf Hermann and The Slime Sisters, and learned their plan to attack Negi. He fought them, stole the bottle that Graf Hermann had been released from, and escaped. However, in an attempt to warn Negi, he was injured and hit with a confusion spell, causing temporarily amnesia. Chizuru and Natsumi, initially thinking him to be just a stray dog, nursed him back to health.

Kotaro has a very lonely past, because being a hanyō has made him, from birth, an outcast of both human and dog-yōkai tribes. (possibly driven from InuYasha as Ken Akamatsu admits to be a fan of that series.) This makes him value both his friendship with Negi, and living with Chizuru and Natsumi, as they become the only true friends or family he's ever had.

Chizuru's maternal attitude to Kotaro (such as her hugging him in public) frequently embarrasses him. He also regularly chastises Negi for being "involved" with so many girls, only to have his relationship with Chizuru thrown in his face (it's a little unclear whether this is because he has a crush on her or whether it's just simple embarrassment). While Kotaro likes to think of himself as a "lone wolf," Negi teased him by saying "You're more like a dog." His views also seem to draw him into arguments with Yue, which he usually loses. He also holds Kaede in high regard, due to her strength.

Like Negi, Kotaro has a somewhat limited view about the world in general, and he is more or less ignorant when it comes to the emotions of girls. His stance on becoming the strongest only in combat leaves him unprepared for things that do not involve fighting.