Self-proclaimed 100-Dan Karate Master.
The student of an unnamed Karate master, Sakaki's teacher instructed him to seek out and join an organization named "Yami" as the master believed the group represented "true martial arts". While developing his abilities and seeking Yomi, Sakaki devloped a strong rivalry and friendship with 2 other karate masters, the reticent Akira Hongo & the more amicable Hajime Suzuki, both of whom also sought Yomi. Hajime suffered from a serious illness that kept his life short, and hoped that before he died he could become the world's greatest practitioner of karate.
When the 3 of them eventually found Yami, things took a darker turn. Yami believed that killing was a fundamental element of martial arts, and only required 1 of the 3 karateka in their organization. Thus they told the three friends to kill one another, and offered the victor a place in their organization. Hajime immediately took Yomi up on their offer and tried to kill the other 2. While Sakaki steadfastly refused to kill his friends, Hongo reluctantly took Hajime up on the offer of a fight to the death and slew him, then squared off with Sakaki. This turn of events turned Sakaki resolutely against Yami and 'the Killing Fist' and he attempted to defeat Hongo without killing him. The battle ended in a draw, and Hongo joined Yami. Sakaki later found & joined the Ryozanpaku dojo, all the masters of which opposed the killing fist as well.
Sakaki's scarred face, short temper, imposing physique and boorish manner mark him as a fairly macho, unapproachable figure, an image further complimented by his love of gambling and drinking, but underneath it all he's ridiculously soft. Despite repeated, loud declarations that he never takes on students, Sakaki is one of the masters most attached to Kenichi, and can become anxious or surly whenever Kenichi is absent or in danger.
Sakaki has no fixed income, but instead makes his money through gambling or infrequent but high-paying bodyguard jobs. Alike Akisame and Ma Kensei, he doesn't just teach martial arts knowledge to Kenichi, but offers personal guidance as well.
As one of the masters at Ryozanpaku, Sakaki can be regarded as one of the most powerful martial artists in the world. As such, he frequently performs feats impossible for ordinary human beings, such as shattering concrete walls, surviving falling from a fatal height, knocking opponents further than humanly possible etc. He uses Karate & is a martial artist of the "Dou" type, meaning that he is someone who uses anger to exceed his usual physical limits.