The kiss could make Yoh more assertive in his feelings for Anna. He might become more protective of her too. As for Anna, she could become softer and more open. For instance, Yoh might start standing up for Anna in front of others more often, while Anna could start smiling more around Yoh.
The kiss might bring out hidden sides of their characters. Yoh could show his romantic side more, like writing poems for Anna. Anna could become more vulnerable, allowing Yoh to see her insecurities. This could lead to a more intimate and understanding relationship between them as they support each other through these new revelations.
It could change how they interact with their friends. Their friends might notice a new level of closeness between them. Yoh could be more affectionate towards Anna in front of their friends, and Anna might rely on Yoh more. They could also inspire their friends to look for deeper relationships themselves.