Short answers, pulled from the story.
Kami is the Japanese word for a deity, divinity, or spirit, written with the kanji 神. It has been used to describe mind, God, a supreme being, one of the Shinto deities, an effigy, a principle, and anything that is worshipped. Motoori Norinaga defined kami in his Kojiki-den as any being that possesses some eminent quality out of the ordinary and is awe-inspiring.
Over 2,446,000 individual kami are enshrined at Yasukuni Shrine in Tokyo. By comparison, the 927 CE codification of Shinto rites listed 3,131 officially recognized kami across 2,861 shrines at that time.
Each kami traditionally possesses two souls: a gentle one called nigi-mitama and an assertive one called ara-mitama. In Yamakage Shinto, a tradition known as Ko-Shinto, two additional hidden souls are recognized, a happy soul called saki-mitama and a mysterious one called kushi-mitama.
Amatsukami are the gods of the heavenly realm Takamagahara, or those descended from the event called Tenson kōrin. Kunitsukami are the gods who appeared on earth in the realm called Ashihara no Nakatsukuni. Susanoo-no-Mikoto and his descendants including Ōkuninushi are considered Kunitsukami.
The four affirmations are: holding fast to tradition and one's family line, having a love of nature, maintaining physical cleanliness, and practicing matsuri, the worship and honor given to the kami and ancestral spirits. Shinto followers observe these to earn magokoro, a pure sincere heart that only the kami can grant.
Sugawara no Michizane lived from 845 to 903 CE and became the kami known as Tenjin, the deity of poetry, after his death. He is an example of an exceptional human being recognized as a kami not for supernatural power but for a distinctive virtue or quality embodied during life. Miniature shrines called hokora have been built in his honor.