The primary male protagonist of the series, Yuu is also the title character, as Miki gives him the "Marmalade Boy" moniker as she thinks that he is like marmalade—pretty looking, yet bitter.

Compared to Miki, Yuu's character is almost a complete opposite, as he tends to be more detached, less emotional, and less open about his feelings. He admits to admiring Miki's ability to show her emotions clearly... and it being one of the reasons he fell in love with her. Numerous people comment throughout the series of how he has a good "poker face." It is shown that Yuu is naturally athletic and smart being able to excel in almost every sport. Yuu's dream is to become an architect and he leaves to attend St. Andrews high school to study it there. Yuu teases Miki a lot, but this is his way of showing affection for her. Although his first kiss with Miki is a complete surprise for her, the Marmalade Boy Movie, which is a prequel to the TV series, reveals that Yuu fell in love with Miki before they actually met. A few days befor the fateful dinner, he saw Miki when she was out and followed her, witnessing her interactions with the Gastman Team.

(Source: Wikipedia, edited)