>>14769463From the time that I had played the games so far, It feels like Serena is warming up to you as the story progress.
At first, she was just doing her neighborly duties and being polite yet distant to you while being the cool,yet somewhat distant to the rest.
I like to think that after the battle in the Tower of Mystery as well as that moment in the cafe in Lumious City is the moment she starts to see you as a rival more than just a neighbor along with being more talkative to you.