Limone (which means lemon in Italian) is a powerful angel sent by Aphrodite to watch over Momoko and the other Love Angels, granting them assistance whenever he is able. Limone is actually a senior angel, just like Momoko's mother Sakura. In the Angel World, he and Angel Lily had a very close relationship.
In the manga, he was always aware of the Love Angels and took a different identity only to remain close to them; he is visibly able to withstand attacks by devils even in that form. He also was scarred by having witnessed Lily's death 100 years prior, but could not reveal the truth to her for fear she would not believe him. She does recover her memories however, and in the final chapter epilogue they are shown to get married.
Yanagiba is Limone's human alias used to watch over the Love Angels. In the anime and manga, Yanagiba is a popular upperclassman at the school and captain of the soccer team. However, in the manga, he remembers everything, while in the anime he has no memory of the Love Angels or that he is one himself until later in the season.