One of the six senior priests designated by Nichiren; he was also known as Sado-ko or Sado-bo. Born in Nagasa District of Awa Province (Mobara Village in Kazusa Province, according to another account) in Japan, he became Nichiren's disciple in 1265. In 1276, when Nichiren learned of the death of Dozen-bo, his former teacher at Seicho-ji temple, he wrote On Repaying Debts of Gratitude in Dozen-bo's honor and dispatched Niko to read it in front of his grave. Between 1278 and 1280, when Nichiren delivered a series of lectures on the Lotus Sutra, Niko wrote those down and compiled them in a work called The Recorded Lectures. After Nichiren's death, he propagated Nichiren's teachings in Kazusa Province, using Myoko-ji temple in Mobara as his base, but neglected to come to Minobu to take his turn in the rotation system for attending to Nichiren's tomb. Around 1285, however, he changed his mind and returned to Minobu, and Nikko appointed him the chief instructor of priests.
Niko did not exercise diligence in refuting or refusing to support teachings that Nichiren had taught were erroneous. He permitted Hakiri Sanenaga, a believer and the steward of the area, to worship at Shinto shrines, donate to the Pure Land school, and engage in other acts Nichiren would never have sanctioned. Nikko cautioned the two repeatedly, but Hakiri preferred Niko's compliant attitude to Nikko's strictness and refused to heed Nikko's warnings. In 1289 Nikko left Minobu to maintain the purity of Nichiren's teachings. After that, Niko became the chief priest of Kuon-ji temple at Minobu. In 1313 he transferred his position to his disciple Nisshin and retired to Mobara, where he died.