A boy who first appeared reading a book that Shizuku had left behind. In the book, Shizuku left her song lyrics inside, and Seiji read them. He, in reality, like them, but when Shizuku returned to retrieve her book, he called the lyrics corny, which caused her to call him a jerk. He's frequently seen riding his bicycle during the movie. Like once when Shizuku forgot her lunch in his grandfather's shop, he returned the lunch to her, with his bicycle. He playfully comments on the enormous amounts of food inside the lunch, asking her if she eats that much. Shizuku again yells at him.
Their first real pleasant encounter is when Shizuku stops Shiro Nishi's (Seiji's grandfather) store while it's closed. She explains to Seiji that she wanted to see The Baron again. So Seiji takes her inside the shop and shows her the cat, he even let's the sun glisten in in the cat's eye. Shizuku is mesmerized by it.
Seiji is an aspiring violin crafter. He even plays a little bit, yet he really loves making them. He wants to become a professional, much to his parent's dismay. He says that only his grandfather supports him. Shizuku pleads him to play a little, and he agrees on one condition. Shizuku must sing along. So the two of them play and sing the song "Country Road". Midway through the song, Seiji's grandfather comes with two of his friends and they join in. This is when Seiji and Shizuku first become great friends.