Instructions
Visit the church on 5 Spring of the first year -- your fifth day in Mineral Town. It̵7;s best to approach the church around noon so as to not miss the first event with Kai, which will give you an advantage when he arrives again in summer. Head toward the confession room and press ̶0;X̶1; to try the door -- you'll be told it̵7;s locked, and then Kai will appear and tell you a confession. Choose ̶0;I̵7;m not Carter̶1; from the dialogue option and Kai̵7;s heart level will go to blue for when he returns.
Visit the beach on the first day of summer to find Kai in his shack. Visiting him every day and talking to him will boost his heart score, and giving him presents will also endear him to you. As with every character in every Harvest Moon game, some presents bring better results. Kai̵7;s easiest-to-find favorites are flour, bread and oil, though he also likes any type of medicine, wine, or any large fish. If you can cook pizza by the first summer or make butter, give those to him every day, too.
Visit the inn one summer night at exactly 8 p.m. to start enough heart event with Kai. Three men will be fighting in the center of the room; approach them and press ̶0;X̶1; to intervene, then choose ̶0;Stop fighting.̶1; Kai will speak to you and his heart level will increase.
Give Kai his favorite items on Summer 22 to receive the birthday affection bonus and to increase your heart score substantially. Then visit the beach on a rainy day in summer once you have bought the largest house you can to activate the final heart event with Kai. After you go to sleep, he̵7;ll propose the next day.