He wasn't sure when he fell in love with her.
First he saw her flags in the morning from his father's tugboat. Some mornings he waved, feeling foolish immediately afterwards. Whoever raised the flags wouldn't be able to see him from their home. Soon his father noticed and began to signal to the flags. At first it was embarrassing, but soon it became part of Shun's routine. He still wondered about the flags. He asked around, but no one seemed to know who the flags belonged to. He busied himself with school, debates, the newspaper, all the usual things.
Then he found out the flags belonged to Umi. Umi was a quiet girl, always very calm and busy. Shun had seen her around but never really took notice of her. Now he watched her from the corner of his eye, looking away if she turned her head in his direction. One night when he had a cough that kept him from sleeping he wrote a poem about her. The next day it fell out of his pocket and was picked up by Shiro, who demanded he publish it in the newspaper. Shun knew better than to argue with Shiro.
The day the poem ran in the newspaper was the morning after the boys found out the Latin Quarter was going to be demolished. Signs were slapped together and plans were made to reveal them at lunch. Shun scrambled around on the roof as signs were unrolled from the windows below, shouts filling the air. Looking back, he wasn't sure what he was thinking when he jumped off the roof. When asked he said it was to garner attention for the clubhouse, though he thought it more likely that he intended for it to garner attention from Umi. After falling through a bush and into a suspicious pool of green water, he was successful. As he shoved his head above water and blew water out of his nose, he was greeted by a clearly concerned Umi. She helped to pull him out the the water, flushing and dropping him back in when someone shouted out a comment about lovebirds.
Later Shiro made a comment about Umi that made Shun think he was at least mildly interested in her. Words were exchanged and Shun was convinced that was no longer the case. That night he cut his hand shaving. His mother scolded him as she helped him bandage it, telling him to be more careful and that he probably didn't need to be shaving anyway. He needed to pay more attention to what he was doing. Shun did not hear much of what she said, his thoughts bouncing between the clubhouse, the mock test for physics, and Umi.
The next time he saw Umi she was in the company of a younger girl who Shun later found out was her sister. The first year asked for his autograph, which he gave freely. His eyes lingered on Umi for a second longer than on her sister. Shiro prattled on about the mock test, drawing both Shun and Umi into the conversation. He proceeded to lead Umi's sister out of the room, leaving Shun and Umi alone together. Shun would have cheerfully killed him.
Time passed in a blur of tests and copying Umi's stencils, her handwriting neater than his ever would be. He gave her a ride on his bicycle and walked with her after school. He saw her standing at the back of the auditorium during a debate, her presence fueling him on as he leapt onto the stage. He spoke with her later and she told him perhaps the Latin Quarter needed to be cleaned. He went to a party at her home and she told him about her father. Worse than the Latin Quarter being demolished was finding out Umi was his sister. He knew he loved her then. It left him feeling like he'd been punched in the gut.
The Latin Quarter was restored and Shun avoided Umi as best he could. Umi's calm presence helped the process immensely and Shiro began to refer to her as their goddess of good luck. Shun was feeling anything but lucky. Seeing Umi was painful, but seeing how how his avoidance hurt her was worse. One day he shared her umbrella in the rain and shared his suspicions. He had to force himself not to look back over his shoulder as he pedaled away, rain pelting his back and soaking his uniform.
Shun threw himself into work at the Latin Quarter, desperately trying to keep his mind off of the girl with the flags. When the students found out the clubhouse would still be demolished, there was uproar. Shiro calmed them as only he could. It was decided that Shun, Shiro, and Umi would go to see the chairman and try to save the house. As they waited on the bench, the awkwardness was tangible. Shiro had confronted Shun about what was wrong between the Shun and Umi, but Shun wouldn't talk. After securing a meeting with the chairman, Shiro abandoned Shun, leaving him to ride on the train with Umi by himself.
He waited with her for the streetcar, the silence between them awkward. Umi turned to him and, in an uncharacteristic fit of passion, professed her love for him. He returned the sentiment without hesitation, watching dumbly as the streetcar pulled off with Umi inside. He didn't understand why love had to be so complicated.
The day of the chairman's visit came, and partway through Shun was pulled out for an emergency phonecall from his father. They spoke hurriedly, emotions racing through Shun as his father told him that Umi's father was not his father and that a man who had known both of their fathers could speak with them both if they came quickly. He pulled Umi out of the house, hope rising up and threatening to choke him. They raced down towards the harbor, time working against them as Shun's heart pounded in his chest. A small boat took them to a large ship. His father's tugboat bobbed alongside, and Shun helped Umi board the tugboat and then the large ship. Together they learned about their fathers, that they were both honorable men and that Umi was definitely not Shun's sister.
The next morning Shun watched from the tugboat as the signal flags were hoisted into the air, his heart swelling. The clubhouse was safe, and more importantly, he was free to love Umi. Though parts of the future seemed uncertain, he had her love. For now, that was enough,