Just look at how Fuutarou and Yotsuba are depicted when he first encounters her in chapter 1. It’s almost as though her bunny ribbon bow wants to reach out and embrace him. Then there’s his ahoge, like a mini version of her bow. For him to be so lost in thought that he didn’t even notice her strains credulity. Something else is being communicated implicitly. Later, we learn that they met each other 5 years earlier in Kyoto, and that day profoundly impacted them both. He thought she was amazing (Raiha said she was his first love) and she’s loved him ever since. She recognized him immediately. He doesn’t know he’s looking at the girl who changed his life, but she goes on to completely change his life a second time over the course of this story. By the end, they’re both in love with each other. Again. I could have gone into much greater detail, but why belabor the point? All of it is foreshadowed in this one panel from the very first chapter. As a final thought, Negi gave us another hint in Yotsuba’s symbol: the four leaf clover. Think about it, Yotsuba is the only quint with a heart shaped symbol. 
