One possible story could be that the girl was really into fashion and makeup. She started by gradually introducing her boyfriend to her world of beauty. She would do his makeup playfully at first, and then she got him to try on some of her more androgynous clothes. Over time, with her influence and his own curiosity, he started to embrace a more feminine appearance and style, eventually becoming more like a girl in terms of his outward presentation.
Well, perhaps the girl was an artist. She saw her boyfriend as a blank canvas. She began by painting his nails, which he initially resisted but then grew to like. Then she convinced him to grow his hair out. She also introduced him to different female - centric hobbies like knitting. Through these small steps, the boyfriend became more in touch with his feminine side and started to look and act more like a girl.
In this story, the girl might be a strong advocate for gender fluidity. She had deep conversations with her boyfriend about how gender is just a social construct. She then took him shopping for women's clothes that she thought would suit him. She taught him how to walk and talk in a more feminine way. As a result of her efforts and his open - mindedness, he transformed into someone with a more girl - like appearance and behavior.