The primary male protagonist of the series, Yuu is also the title character, as Miki gives him the "Marmalade Boy" nickname because she believes he resembles marmalade—visually appealing but with a hint of bitterness.
In contrast to Miki, Yuu's character is almost entirely different. He tends to be more reserved, less emotional, and less open about his feelings. He admits to admiring Miki's ability to express her emotions clearly, which is one of the reasons he fell in love with her. Throughout the series, many people comment on how he has an excellent "poker face."
Yuu is naturally athletic and intelligent, excelling in almost every sport. His dream is to become an architect, leading him to attend St. Andrews High School to pursue his studies. Although he often teases Miki, this is his way of showing affection for her. While their first kiss comes as a complete surprise to Miki, the Marmalade Boy Movie, which serves as a prequel to the TV series, reveals that Yuu had already fallen in love with Miki before they officially met. A few days before their fateful dinner, he saw Miki out and followed her, observing her interactions with the Gastman Team.
