Yuki is the charming young man Chiharu Nitta met during summer training camp. He is a city slicker with a beat for nature and lots of hobbies. At first, Chiharu shrugged him off, but because she was lonely and in a fight with Kyoichi, she nearly gave in. Though he already had girlfriend waiting in the city, he continued his advances and managed to make Chiharu fall for him. They spent a night together at a lookout point (it is unknown if they actually had sex but it is very much presumed). She gave him a goodbye kiss which was witnessed by Kyoichi.

(Source: Wikipedia, edited)