Mikao Usui was born into a samurai family and was a descendent of Toshitane Chiba (a warlord who conquered the city of Usui and took that city for his name and the name of all of his family). His family were Tendai Buddhists and Usui studied in a Tendai monastery when he was a young child.
At the age of twelve, Usui studied martial arts (aiki jutsu, yagyu ryu).
He studied history, medical books, Buddhist scriptures, Christian scriptures and was proficient in psychology, Taoism, divination, incantation and physiognomy. He travelled widely through Europe, China and USA.
Usui ended being a zaike, which is a lay Tendai monk. This meant that he could be a family man, living at home with his wife and kids, and not being required to live in a temple with other Tendai monks.
One day, Usui decided he wanted enlightenment (although, as a lay monk, he had probably been working towards this his whole life). But this time, he was serious – it was enlightenment or die.
So he climbed up Mount Kurama and did a severe discipline of fasting for twenty-one days. On the last day of this practice, “he suddenly felt One Great Reiki over his head and attained enlightenment and he obtained” Reiki (excerpt from the Mikao Usui Memorial Stone, 1927).
This One Great Reiki allowed him to attune people with just his eyes (no symbols needed), while the rest of his students-turned-masters needed to draw symbols for attunements. Usui ended up teaching over 2000 students.
Now, being a monk, Usui used Reiki very much as a tool for spiritual growth and encouraged his students to use it this way too.
Dr Usui died of a stroke in 1926 at the age of 62. He was cremated and his ashes placed in a Tendai Monastery in Tokyo.