Kōsa (光佐?, February 20, 1543 – December 27, 1592), also known as Hongan-ji Kennyo (本願寺 顕如), was the 11th head of the Hongan-ji in Kyoto, and Chief Abbot of Ishiyama Hongan-ji, cathedral fortress of the Ikkō-ikki (Buddhist warrior monks and peasants who opposed samurai rule), during its siege at the end of the Sengoku period. He engineered many alliances, and organized the defenses of the cathedral to the point that most at the time considered Ishiyama Hongan-ji to be unbreachable.
In 1570, Takeda Shingen, a relative of Kōsa through marriage, faced not one but three major rivals: Oda Nobunaga, Tokugawa Ieyasu, and Uesugi Kenshin. He asked the Abbot for aid, and Kōsa persuaded the Ikkō sectarians (also called monto) in Kaga Province to rise up against Uesugi Kenshin. Several years later, after the death of Takeda Shingen, Kōsa secured the aid of the Mōri clan in fighting Oda Nobunaga and defending the Hongan-ji's supply lines from blockade.
Oda Nobunaga's Siege of Ishiyama Hongan-ji began in 1570, and would be the longest siege in Japanese history. In accordance with Kōsa's strategic organization of the defenses and alliances, the fortress was all but impervious to attack, and the Mōri clan fleet defended the supply lines for some time. Kōsa (and the Mōri) requested aid from both Takeda Katsuyori (son of Shingen) and Uesugi Kenshin, as well as the Ikkō armies of other provinces, to attack Nobunaga, relieving the siege. But neither Takeda nor Uesugi answered his call. When Nobunaga attacked the fortress with 3,000 troops, the monks answered with 15,000; Nobunaga then turned to attacking Kōsa's allies, the Asakura, Uesugi, and Azai clans as well as other outposts of Ikkō-ikki monks. The Abbot wrote a desperate letter to Ikko followers in Musashi and Sagami Provinces, asking them to stand fast, and to send supplies and reinforcements. The fortress' supplies were nearly exhausted, and their outposts were nearly all destroyed by 1580. Nobunaga ordered the Abbot to evacuate the fortress, and to leave Osaka. Kōsa deliberated with his allies, particularly the Mōri, and then left for Kii Province, hoping to raise reinforcements. He left his son in charge of the fortress, who surrendered after an Imperial Messenger arrived with an official request from the Emperor that he do so. The fortress mysteriously burned down after the monks left it.
Despite this incredible defeat, Kōsa remained devoted to the Ikko sect, and to the idea of regaining a central cathedral fortress for the sect. He began to enlist the help of Ikko sectarians to aid Toyotomi Hideyoshi, in order to gain Hideyoshi's favor. In 1583, Kōsa sent Ikko warriors to harass Hideyoshi's enemy Shibata Katsuie, and in 1587 he sent messengers to Kyushu, asking Ikko leaders there to act as guides for Hideyoshi's army in Satsuma Province. In 1589, Hideyoshi granted Kōsa his wish for a new Hongan-ji. The site was the shrine to the patriarch Shinran, in Kyoto; it was moved in 1591, and is now known as the Nishi Hongan-ji.