You do have to give any girl you want the lounge events for enough killing blows. On my first play through, I had really leveled up the senior captain Alpha, but I didn’t get her date event first time through. Second play through, I gave her the kill shots on the Yoju with the captured humans, rather than as my clean up hitter on the general rabble, and her event popped up quickly.
There seems to be a minimum number of required “boss” kills, as well as a general level up required to trigger those events. Some soldiers thresholds are just very low, because they start off so low in level. Others, like Alpha, require more, because they start at a higher level (making them harder to level up to meet the gained level requirement).
Won’t Peach Princess send you a cleared 100% saved game file for their games when walk throughs won’t work because it would be “too general” like PSW)? I know they’ll send you a walk through for any of their titles, but I don’t see a walk through working for PSW, since it’s just basic tactics. To get a game walk through from PP, just email them and ask for one.