Claire is needed for the harem route. My reasoning for this is that she's known the girls for much longer than MC, has earned their trust, and has better insight on how to convince each of them to join. Of course, she needs MC as well because she doesn't have what the girls are necessarily attracted to.
Elizabeth is on the backburner for now, but she will eventually have her own update that adds additional paths throughout the game and her own ending
Claire
is the one to first proposition you for the harem, but in the coming Chapters I hope to show that MC gets more confident with the idea and starts to come into his own. As for Claire's overall behavior, I'll only say there's more to learn about her motivations and backstory later in the game!
You will not be able cum inside any of the girls the first time you have sex
That's something I'd like to build up to