A good introduction to Mikao Usui can be found on the pages of Reiki Guild
Here is an extract:
Mikao Usui rediscovered the system of Reiki in Japan at the end of the 19th Century following a lifetime of study, effort and dedication.
His life-changing experience came through a meditation retreat on Mount Kurama near Kyoto in 1922. From this he developed the simple system of Reiki that we know today.
Following the huge 1923 earthquake which ripped through Tokyo causing firestorms and destruction on an almost unknown scale, Usui and his students gave Reiki to countless suffering people. Their dedication and compassion meant that his Reiki system became known to many people who studied.
Through Mikao Usui’s teaching career, he taught many people Reiki. They in turn, passed this system to their own students.
For people particularly outside of Japan, Reiki was spread through a retired naval officier – Chujiro Hayashi who was a prolific teacher. One of his students, a Hawaiian Japanese lady called Hawayo Takata who was a widow and single mother, was singlehandedly responsible for the spread of Reiki from Hawaii to the US and from there around the world.
Be full of gratitude
Work hard on yourself
Be kind to all people