One common theme could be love and affection. Since it involves a 'beso' (kiss), the fanfic may focus on the deepening relationship between Ash and Serena.
Love and relationship development is a very common theme. Most fanfics focus on how Harry and Ginny's relationship grows, from initial attraction to deep love.
There are many fanfic platforms out there. Some popular ones apart from Archive of Our Own are FanFiction.net. Browse through their Pokémon section and look for stories with Ash and Serena in the tags. Then, you can skim through the descriptions or reviews to find the ones that might have a 'beso' scene as you desire.
There's 'A New Beginning for Harry and Ginny'. This fanfic focuses on their relationship from Ginny's perspective. It shows how Ginny's strong personality influences their relationship dynamics, and it has some really heart - warming moments between them.
Perhaps 'y' is a relative of Harry's that was thought to be dead. This relative could have a unique magical heritage that they pass on to Harry, making him even more powerful. For example, if 'y' was a distant cousin from a line of powerful seers, Harry might develop some precognitive abilities.
After the first beso, their relationship might become more confident. They start to be more open about their feelings for each other. They hold hands more often and share more affectionate glances.
Ginny might support Harry by simply being there for him. In the face of everyone else turning their backs, her presence can be a great comfort. For example, she could sit with him at meals when no one else will.
One possible reason could be miscommunication. Maybe Ginny feels neglected by Harry because he's so caught up in his own heroic deeds and doesn't pay enough attention to her. So she might seek comfort or excitement elsewhere.
Their relationship could become more spiritual. Without the physical limitations and distractions of the living world, they can focus solely on their emotional and mental connection. They might be able to communicate with just a thought and understand each other on a deeper level.