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Takashi Kosuda is the primary love interest of Yamada, who was using him as her stepping stone to realizing her goal of having 100 sex partners, by taking advantage of his virginity. Despite being plain-looking and characterized as inept, sheepish and indecisive, he is kind-hearted and considerate and tries to look out for Yamada whenever he could. He is also very scared of women. He is 17 years old.  

He is initially taken by her advances but later turns into confusion and anxiety, due to mixed signals between them as well as his insecurities as a virgin. He is also considered dense (one is when Mayu Miyano gives him a chocolate cake as a sign of her crush and love for him, but he mistakes it as a token of thanks for the apples given to her).

He lives at home with his elder sister, Kazuki Kosuda, who is a college student and often cares, protects and encourages him to continue his relationship with Yamada as well as giving advices occasionally. One of his hobbies is photography. He is also afraid of heights. Kosuda also has a sex god, who was in a lazy mood, and dressed in pajamas. He is helpful (as seen helping several people as a comedy when he missed his train and stuck with several people in need of help).

He is also often with his friends with unknown names(although their surnames are shown in both manga and anime, the one with the spiky hair being called Akimoto and the one dressed in blue sweater known as Kudou) and have minor roles and are as plain-looking as him. At the end of the series Kosuda establishes a relationship with Yamada, and they finally have sex. The two two begin dating each other soon after.