Myouren was actually a real Buddhist priest born in Shinano Province, which is ZUN's hometown, and trained at Chougosonshi Temple (朝護孫子寺) on Mount Shigi (信貴山). He appears in several stories, but the most significant one is Shigisan Engi, which is one of the most popular emakimono. In its third volume, there is a Buddhist priestess, whose name is unknown. She wants to meet Myouren, who is her younger brother. She is the inspiration for Byakuren: she was looking for Myouren because she was worried about him not coming back, so she stayed at Toudaiji, where Myouren had made the vows for priesthood. Daibutsu tells her in a dream that there is a purple cloud over the side of the mountain southwest from where she is. She went the next morning, and finally she was able to meet and live together with him.
But it's quite important, but in the setting text, there's no description of "Myouren Hijiri" ― concerning the real him, "Myouren" is a dharma name (kaimyou, 戒名), which buddism priests get after entering shrines. And according to Japanese Wikipedia's kaimyou, buddhism priests originally don't have their family names, because they have thrown away their vulgar parts, of course also their family names, to be priest. They've called theirselves with family names regularly since later Muromachi period, approximately since 1500 C.E., but the real Myouren is a figure of middle Heian period, approximately of 1000/1100 C.E. So the real Myouren should not call himself or be called "Myouren" with his family name.
In an interview with ZUN, he states that Myouren was originally meant to be the final boss of UFO, but he decided against it because "It would be weird to have an old man as the last boss in Touhou".
It is implied by Murasa after you beat her in the SanaeA route that he is the one that left the grain silo that is used to revive Byakuren.