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.

Seiji later gets the chance to go to Italy and study the art of violin crafting. He first tells Shizuku, and thus her realizing that they're going to be apart for two months. Shizuku envies him for being able to plan out his future and his dream. So she decides to find out her own dreams. She decides to become an author after hearing Seiji's real opinion on her song. So writes about the Baron.

After Seiji's two month trip, he returns and meets Shizuku early in the morning. On his bicycle, he takes her to a special place, a great scenery of the morning sun rise. Seiji explains that he'll be finishing his Junior High days of school, and then will be returning to Italy. But he promises Shizuku that he would return. And he asks her, during the morning sunrise, to marry him when they get older. Shizuku doens't hesitate to say yes, which overjoys Seiji.