After you finish the main quest, it hints that you should speak with Serena and she mentioned staying at her own place. Where is that? I think I have looked in all the houses and cant seem to find her?

The building you and her sister woke up in after the flower fight, with the prison cells. You can change her outfit, talk to her, have sex and even party up but the next part of the story is in the basement of the power plant and involves two new characters you haven't met yet.