The Hakurei Shrine sits on the border between Gensokyo and the rest of Earth proper, existing on both sides as the anchor of the Great Hakurei Barrier and accessible from either side, though the Earth-side version is said to appear old and abandoned. Outside world items appear on the grounds from time to time, but no one knows how to consciously cross from one side to the other save for Yakumo Yukari and Reimu, the latter of whom will send outsiders home if asked.
Ibaraki Kasen, Ibuki Suika, Yakumo Yukari, and Reimu's best friend Kirisame Marisa are frequent guests. The old turtle spirit Genjii and the Three Fairies of Light (Sunny Milk, Luna Child, and Star Sapphire) live in a spring and a tree behind the shrine, respectively. Mima once haunted the grounds, but has been absent from the games since the PC98 era and is only assumed to still be present. The inchling princess Sukuna Shinmyoumaru also lived at the shrine in a converted birdcage for a period of time. Clownpiece has taken up residence in the basement since the events of Legacy of Lunatic Kingdom. And others (Hinanawi Tenshi, Yorigami Shion, etc) have also taken up temporary residence over the years. The patron god of Hakurei Shrine is unknown, even to Reimu, though theories include Yukari, Mima, and the Great Dragon.
