He is the Protegee of the Legendary Onmyouji Abe no Seimei, and quite powerful in his own right, he is well known all over Kyou for his great ability to Exorcise demons or youkais. Unlike most of the Hachiyou, Yasuaki was not born a human. He was created through the powerful magic skill of the Abe no Seimei, made into his current form through the same ability in the manga, which is not explain ed in the anime. Despite having a physical appearance of a twenty year old adult male, he has only been alive for at least two years. It explains his lack of emotional simplicity and has a naivety of a child, since he has a lot to learn about many things that his allies and companions have grown up learning. Yasuaki is very composed, and detached person that runs on utter logic and sometimes can be seen as a cold or a strange one. For example in the part where Eisen will die if he were to purify or exorcise the eight stringed Kin, he said that without any concern that is simply black and white and to save the Miko, they as the Hachiyou, should be prepared to do his duty, as also states flatly that "If there is no Miko, then there will be no point of the Hachiyou."
Yasuaki lacks a heart, and the Seimei created him to find someone important and the heart which could not be made. He believes himself not to be a real human being, and and does not fit in the same way as the companions. He believed that he is a tool for the Miko, which in the manga he was concerned with the thought that he is 'broken' because he could not read Akane's heart. Which, Akane had contradicted that, and explained to him that it was normal and makes him every other person, and he was not broken at all. He believes that he has no heart, but it is proved that he has one fairly early on, when he cast a spell on a dead branch and make it a flower to cheer up Akane, who was crying. It is believed that at the moment he received his dragon jewel, that his emotions, and his 'heart' stirred inside of him. He has the odd ability to speak to trees, and uses animals to convey messages and even uses dolls to help him do cleaning tasks.
Even though he seems to be the most powerful of the Hachiyou, really he is the most vulnerable one, seeing his birth, and his origins. Yasuaki is quite naive like a child, when he questions why people lie, and he himself lied about Kotengu's death and it tortures him greatly believing that she was definitely angry with him for lying to her like he did. Yasuaki also has a habit of saying "Mondai Nai" (No problem) and sometimes "Wakaranai" (I don't understand) when relating to some of the human nuance that he has yet to grasp. Yasuaki has no need for formalities, and usually addresses people informally, even the miko, though the other Hachiyou uses her in a more respective way, as well as sometimes he addresses to Akane as "Omae" (you) and never using her name, not even when he confesses his utter secrets to her. Even though he does seem like one of great knowledge, Yasuaki lacks human ways, and emotions, and has trouble displaying them or understanding them completely. It shows that he is indeed only two years of age, that he has a lot to learn about being a human.