My impression was that the different attitude from Jeremy comes more from him being with Louise and Lena is known by Ian / Ivy as well... so even if he want to act like his usual player attitude, he thinks that he mightl lose something if he openly pursuit Lena (Louise, chances with Ivy, or even Ian friendship depending of the route).
Kind of like he assumes that Lena is out of the field, for the moment, if he wants to keep things going with his other conquests.
The party might as well open a way for him to pursue her more agressively due to her being open of what she wants in that path.
Lena attitude towards Jeremy is purely sexual, if you compare the times that she fantasizes about someone (Taking for example Axel, which with him is all about his attiude and her role, and Jeremy is just the dick), seems that its written that way un purpose, on personal level they have no chemistry besides the fetishes, like him being Louise boyfriend or having a huge dick.
Their interactions outside the party are quite "cordial" or passive/agressive (Again, depending on which path are you), so it seems that the path for both of them needs a lot of work yet (Like... having some common ground besides sex) or go towards the usual rabbit hole and have a purely sexual relationship.
It give me vibes of GGGB that his role will be closely tied to Ivy and Louise in sharing / swapping / cheating scenarios more than a romantic interest due to his personality (for the moment) being something that Lena starts the game not being fond of.
I think that makes a lot of sense, also in comparison to GGGB. The part I’m most unsure about, is how Ian’s potential influence on Ivy will be, but maybe we’ll see some further interactions between them in the coming chapter.
I suppose that this chapter we might see more of this, because for "new" relationships only Ivy and Louise are left to determine which kind of interaction will Ian have with them.
I hope for Ian/Ivy interaction had the possibility on going both ways; ian delving in Ivy's world due to him being drawn by her and he being "corrupted" in a way ; and the other way, the opposite, like Ian being the kind of guy that was teased at the start of the game that might rock her world hard and make her root for a long relationship (Lena will have this role as well in other path, for sure, i hope)