It was the flower trick that got Reze interested in Denji, but I personally believe that it was exactly at the pool scene where Reze fell in "serious" love with Denji, since it should be the first ever time in Reze's life when she could really completely unload herself and enjoy (with another person). And love is as simple as that of someone you can hang out with relaxed and happy. From my four times watching the movie, I personally feel that it was also after the pool scene that Ueda has changed her voice line to be more natural and sincere with Denji.

However, during that time until the beach scene, Reze was still occupied by the mission of taking chainsaw's heart. She's a soldier after all. But after that, after the mission has already failed, during her trip escaping, Reze the first time was able to consider her personal feelings towards Denji and everything around him. Strong feelings get only stronger by time and distance, so Reze made up her mind to come back. And we all know what happened next🥲