Tanbo de Hirotta Onna Kishi, Inaka de Ore no Yome da to Omowarete Iru

Jin Mita, a farmer in the countryside, discovers a female knight lying in the rice paddy. When he asks the knight, who calls herself "Seraphim," about her story, it seems that she came from another world. The female knight, suddenly thrown into modern Japan, naturally has nowhere else to go, so Jin takes her in, and in exchange she'll help with his farm. Jin teaches Seraphim, who has no understanding of this world, about agriculture, modern society, and life in the countryside. However, they look just like an ordinary couple to everyone around them....... Before long, people all treated Seraphim like she's Jin's wife...!

(Source: Dengeki Bunko, translated)