Kekkon shitai Ryuugu-san wa Jouriku shimashita

The handsome college girl "Nii-san" rescued a turtle and was taken to the Dragon's Palace, where Otohime-sama immediately fell in love with her. Otohime-sama chased Nii-san ashore, and her single-minded love pierced Nii-san's heart as well! Common sense doesn't matter in the face of this love!