In fanfiction, when Ash and Serena meet again, they might share a warm embrace. Maybe Serena has new Pokémon skills to show Ash. They could also start a new adventure together, exploring uncharted regions and facing powerful new opponents.
Maybe Ash's character in the original work has a certain fixed image, and some fanfics about Serena deviate from this image he represents, so he hates it.
Maybe Ash is portrayed in a way he doesn't like in Serena fanfiction. For example, the stories might change his character traits or put him in situations he wouldn't be in the original series.
In many Pokemon fanfictions, Ash might comfort Serena by simply being there for her. For example, if Serena is feeling down about a failed performance in a Pokemon Contest, Ash could offer words of encouragement like 'You'll do better next time, Serena. I've seen how hard you practice.' He might also share his own experiences of facing setbacks in Pokemon battles to make her feel that she's not alone in facing difficulties.
One possible reason could be a miscommunication. Maybe Serena did something that Ash misinterpreted. For example, she might have made a decision about their pokemon journey without consulting him, and Ash, being independent, took it the wrong way.
Serena's character development could be profound. First, she'd probably experience a great deal of grief. But through that grief, she could transform. She might become more fearless in battles, using Ash's sacrifice as motivation. Also, she could develop leadership qualities. Since Ash is no longer there to lead in some situations, she has to step up. She may also form new alliances and friendships as she tries to cope with the loss and move forward in the Pokémon world.
They would probably be a bit shy at first. Ash being his usual energetic self but also respectful of Serena's space in the tent. Serena might be a bit nervous but also excited to be sharing such a close space with Ash.