Reiki was founded by Usui Mikao in the early 20th Century in Japan as a non-sectarian method of energy healing. At the time Reiki was founded the Japanese government was cracking down on shinshukyo (new religions in Japanese). In order to avoid government scrutiny Usui made Reiki into a commercial enterprise - entirely bypassing many of the assumed hallmarks of a spiritual tradition. While the government bought the idea on the surface, they did try to force Chujiro Hayashi to teach the military - he committed suicide to maintain his honor as that was an order he could not follow.
Japan had a number of native Shinto traditions and various branches of esoteric Buddhism including Shingon and Tendai. Usui was a Tendai lay monk - that is to say he went to the equivalent of Catholic school in Japan. He held a number different jobs until he achieved Reiki in the early 1920s on Mount Kurama. The Reiki Gakkai was setup in 1922 and Usui himself attracted much attention after the Great Kanto Earthquake of 1923. He diligently applied himself to applying healing and compassion to thousands of people after the disaster occurred.
Whatever influences he may have had it is known that Usui was familiar not only with Tendai Buddhism, he also had connections to Joh-Rei, Onmyodo, Oomoto-kyo samurai aiki arts, Shingon, and Shugendo. After his 21 day fast on the mountain it's said he achieved 2 out of 6 levels of the Reiki system and that others will achieve much more than him in the future. Usui Mikao was not an adept, rather he was walking well worn paths to awakening that were empowered by kaji and various adepts. Our traditional Japanese Reiki stems from the Hirai family and from the Jikiden branch of Reiki.
Our Western Usui Reiki descends from many of Hawayo Takata's students and our overall system would be considered Usui Tibetan Reiki as made popular by William Rand. Matt also has attunements to Shamanic Reiki made popular by Jim PathFinder Ewing. Dragon and Garuda Yoga would like to express gratitude to Brett Bevell for sharing Usui's Psychic Reiki Crystal - it has been a priceless addition.
The material made by Yolanda Williams at The Energetic Alchemist has been incredible for deepening the actual practice of the 5 Precepts and integrating concepts of intuitive development into my traditional meditative practices. Phoenix Rising Star's attunements, Healing Angel Protocol is another valued system that we feel dovetails well with Reiki.