Sei Brunehilde Gakuen Shoujo Kishidan to Junpaku no Panty THE ANIMATION

Based on the erotic game by One-up.

Kabuto comes from an old clan with many customs, one of them being arranged marriage. Thus one day, while attending boarding school, a woman named Miyako Ruriko appears to Kabuto and introduces herself as his fiancee. While Kabuto isn't too eager to follow the traditions of his clan, Miyako is infatuated with her fiancee and wants to prove herself as an adequate wife, therefore lewd Kabuto decides to put his fiancee to a test...