Kosanta has two older brothers, Matashige and Akugorou. Kosanta went missing in the quest, Brotherly Bonds, and his brothers were scared to think that he had been kidnapped by Oni-basami, the boss of Hosuseri Shrine. Upon being found, he explains that the letter left by himself was written as him hanging out with Oni-basami, when in fact it actually was written as "Help! I have been kidnapped!" This can be seen at his absent-minded nature due to the simplicity of him writing: I have been kidnapped" instead of "hanging out with a friend". He is still friends with Oni-basami, as he takes care of his wounds after the Oni who went to look for him, the player, beat him up.
Unlike most NPCs, Kosanta is unable to be found until the quest, Brotherly Bonds, is started. Once the quest is started he can be found at the end of Hosuseri Shrine dungeon right in front of the shrine. Once you talk to him and return to his older brothers, Akugorou and Matashige, he will then be permanently next to his older brothers in the North-West part of Oosumi Plains West.