Though generally regarded to have been called "Kumagai" in actual historical fact, Naozane is almost always called "Kumagae" in theatrical productions and other fictionalized works.
Kumagai was born in Kumagai village, in the Ōsato District of Musashi Province. He lost his father at a young age, and was raised by his maternal uncle, Hisaka Naomitsu. When Minamoto no Yoritomo first raised his army, Kumagai sided with the Heike (Taira) clan. However, he soon switched allegiances to the Genji (Minamoto) clan. Kumagai's most well-known battle was at Ichi no Tani, where he confronted the Heike prince, Taira no Atsumori.
The Noh play Atsumori describes Atsumori's death as follows: The death of Atsumori is describned as follows:
Because Atsumori's name was not familiar to him, Kumagai then knew that Atsumori had never been in a battle before. After taking the time to look through Atsumori's belongings, he found a certain flute, known as the "Saeda" (little twig). Discovering that the flute was given to Taira no Tadamori by Emperor Toba, and later passed down to Atsumori, he reportedly felt even more sadness and regret for his actions. The night before the battle, while Kumagai was in his tent, he had heard someone playing the flute with skill outside the enemy camp, and he now realized that that flute player may have been Atsumori.
Later in life, Kumagai remained remorseful over the people he killed in his career as a soldier, and grew discontented with following Minamoto Yoritomo, so he visited the Pure Land Buddhist monk, Honen and explained that he was concerned with the afterlife. Honen told him that he need only recite the name of Amida Buddha, the nembutsu, and that regardless of his former life, he would be reborn in the Pure Land. Kumagai was said to have burst into tears, fearing that Honen would instruct him to "cut off his hands and feet or give up his life" in order to find salvation from his sins.
From there, Kumagai became a close follower of Honen and Jodo Shu Buddhism, and took the ordained named Rensei. Letters are still preserved between Honen and Rensei, where Honen advises Rensei to continue reciting the nembutsu, and to look after his aging mother. Rensei later made a written vow before a statue of Amida Buddha, stating that he would strive to reach the highest rebirth in the Pure Land, as depicted in the Buddhist text, the Contemplation Sutra:
As a close disciple of Honen, Rensei became a popular instructor on Pure Land Buddhism and helped spread the new doctrine to others who also became Honen's disciples. In the summer of 1206, Rensei/Kumagai passed away one morning while repeating the nembutsu over and over in front of a hung image of Amida Buddha and his attendant Bodhisattvas.