In Japanese esoteric Buddhism, buddhas and bodhisattvas are often represented by single character "seed-syllables" written in bonji, a script based on Sanskrit. On top is the seed-syllable for Dainichi Nyorai of the Womb Realm. (There are two realms in esoteric Buddhism, the Diamond Realm and the Womb Realm.) Dainichi Nyorai is the Cosmic Sun Buddha, encompassing all aspects of existence. As the inscription says, Dainichi is unborn, and is not past or present. Dainichi simply is.
Jiho Hyakunyo was a Tendai scholar-monk. After training on Mount Hiei, Jiho served as abbot of several Tendai temples, and then spent the last years of his life as a hermit. Jiho was a scholar of bonji (Sanskrit), a poet, a calligrapher, and a practitioner of nembutsu, chanting the name of Amida Buddha.