Seimei possesses untrimmed hair falling to his chin and striking golden eyes. His attire consistently features green, combining a shoulder cape over lighter green garb with visible orange underclothing.

His abilities stem from the Chronometer, a time-manipulating artifact. During season 2 events, his powers were completely absorbed by this device. This absorption triggered two profound changes: the Chronometer attained sentience and gained the capacity to repair itself even after physical destruction. Consequently, Seimei retained only a faint ability to sense the Chronometer's presence. He theorized that others, such as "Mugensai the Second," might possess similar sensitivity.

In season 1, Seimei resided at Tokugawa Yoshinobu's residence and was present during a prisoner rescue operation. Later, in season 2, he appeared to alert another character to supernatural disturbances, specifically connecting reports of ghost lights to the Chronometer's activity which he sensed. He also perceived an impending large-scale disaster coinciding with mysterious sparks igniting fires across Kyoto.

Earlier, during season 1, Seimei required rescue by other characters after being held captive on Prison Gate Island.