Well...sayonara = good byeja ne, or just "ja" = see you later (a less formal goodbye, probably more suitable for a boyfriend)So that's out of the way. Now, boyfriend... "kareshi" or "kare" (pronounced "KAH - reh") is boyfriend, and "sukidoushi" works for boyfriend OR girlfriend, as well as general "soul mate". In other words,"Ja ne, kareshi." = See you later, boyfriend."Ja, kare." = See you later, boyfriend."Sayonara, sukidoushi." = Goodbye, soul mate.Any combination could work. However, I assume most girlfriends would simply refer to their boyfriend by their name and a familiar honorific, such as "-kun".(Note: I'm not certain on this, but "kareshi-kun" or "kare-kun" might be a fine way to say boyfriend as well, while being more familiar. But don't quote me on that.)