Yakuma has narrow blue eyes and long black hair, which he keeps wrapped up in a ponytail with bandages. He wears a white hat with a blue sun-like pattern on it. He later cuts his hair up to shoulder-length. It is noted by several that he is quite beautiful and feminine looking, in contrast to his straightfoward and tough personality. While visiting the Village of Brides, he thoroughly refused to disguise as a female with comic results - as it turned out, he didn't even need a disguise. The women of the village mistook him for a woman anyway.
Yakuma is a serious, honest, and trusting person. No matter how many times he is tricked, he still trusts the person he was tricked by. He claims he hates Itsuwaribito, but still thinks they don't deserve to die. He is against lying, and states that he will live a righteous life. As a doctor, he states that it is his duty to examine a person a many times as it is required, and that it didn't matter if he was being tricked or not.
Despite his serious personality, he also has a softer, more joking side to him which he often shows to Neya and Pochi. He is exceptionally perceptive, able to tell that Neya liked Utsuho when she herself had yet to realize it. He once teased Neya about her weight and her feelings for Utsuho, adding that he would always support her. It is hinted that he could tell that Neya was in love with Utsuho because Yakuma himself is in love with somebody from the capital.
A running gag throughout the series is that almost all characters who come in contact with Yakuma at least once refer to him as a mother. He often scolds Utsuho and Pochi, usually accompanied by "And don't get hurt!" Pochi once said, "I lived with Mom in the mountains! Even though she's not around anymore... with Utsuho-san, and Nee-chan (Neya), and another Mom (Yakuma), I'm so happy!" Pochi is the only who Yakuma allows to call him 'Mom' freely.
Yakuma and Utsuho are like fire and water: Yakuma lives for the truth and trust while Utsuho lives for lies and deceit. Yakuma is usually the victim for Utsuho's jokes, always getting kicked or punched by him or being tricked into paying the bills. However, the two get along surprisingly well and Yakuma acknowledges that while Utsuho acts as if he only cares for Pochi, Utsuho does in fact care about both him and Neya. He sometimes admits to himself that Utsuho is an amazing person. Oddly enough he is the one who understands Utsuho the most, even able to understand his reaction towards Neya's confession. He scolds Utsuho and Pochi like a mother would: warning them to stay out of trouble and not get hurt.
Yakuma shows a softer side of himself towards Neya, who he cares for like a sister. He is the very first to recognize Neya's feelings for Utsuho, even before Neya herself realized them. He sometimes jokes around or teases her, namely about either her weight or Utsuho. He also says that he will support her no matter what, and encourages her to not give up. Unfortunately, Yakuma is also the mistaken target of Neya's abuse; Neya often tries to hit Utsuho or throw something at him, only for Utsuho to dodge and Yakuma gets hit instead.
Pochi views Yakuma to be his surrogate mother. Yakuma is always scolding both Pochi and Utsuho to not get hurt or get in trouble. He also tries to tell Pochi not to always believe whatever Utsuho says. Yakuma always treats Pochi's wounds and lets Pochi sit in his lap. The relationship between the two can only be described as mother and child.
Born to a single mother with an unknown (white) father he was always fascinated with medicine. He read over books and taught himself. One day his mother falls ill and he almost resorts to treating her himself before a doctor from overseas enters the scene and treats his mother.
After this Koshirou shadowed the man and begged to learn from him, something he always refused. That was until his mother's death when he stowed away on the ship to the Netherlands. He was finally taken on as the apprentice of Yuni Conrad.
He was taught by this man and eventually grew into a skilled doctor.
Yakuma's goal is to help the Onkado. He travels to search for a cure to the unknown disease that the person has, and to find others with the same disease. He is very content to saving this person, and he even went to an island of exile simply because there was a small chance a medicine that gave a person eternal life was there.
Despite his young age, Yakuma is a skilled doctor and is one of the few who is able to perform surgery.
Yakuma's fighting skills are almost on par with Utsuho's, namely his strength and speed. However, his fighting style is almost ridiculously straightforward as he usually just charges in head-on. Later he adapts to pulling a few tricks (cutting off his hair and attaching it to a log as a decoy) though he hates himself for it afterwards.
He is fluent in dutch.
The Sapphire Magata
While in possession of his treasure Yakuma possessed the ability to preform quick surgery. (reattaching limbs in an instant when they had been severed by gin) Though it is revealed to have a great toll on his body.