shorter analytical post abt things I remember very clearly. tangle tower spoilers under cut
so like. penny. penelope. what I find really compelling about her is how the narrative sets her up as foil to both freya and poppy.
with freya, it's obvious. penny, despite her efforts, cannot free herself from tangle tower & her family at large. they both lived in the outskirts of their respective families... neither of them had their parents in their lives. penny, as pointed out by felix, was raised by her uncle (professor pointer). the timeline of the ambassadors is... very very fuzzy, of course, and hard to make out - but we can assume that penny's parents left very early. the study was still in use when penny was 19, so clearly they did not all leave then, or perhaps they intermittently came back to make use of it in secret. whatever the case, penny was essentially orphaned. freya, too, was not raised by her parents. both of them wanted to leave.
but penny's driving force is her desire to have what she feels she is owed. she was abandoned by her family, desperate to have something to call hers, bitter at the ambassadors for seemingly rejecting her. at the very least, their respective missions were ones she was disillusioned with upon finding out. it's implied she was upset enough to kill them, upon finding this out. freya does not have a similar drive. she wants to forge her own path. she wants to free fifi from an inheritance she doesn't want. I'm not entirely certain she would have exposed the family publicly... I think she was likely planning on simply telling fifi privately. this is purely speculation, though, based on the bits of freya's personality we hear about. I don't think she'd want flora to be reminded of her grief like that.
with poppy... that feels fairly obvious to me, too. penny seems deeply bitter towards her. and it feels clear why. penny's obsession with what she is owed, what was taken from her... of course poppy is a target of her ire. poppy, who got all the love from professor pointer, again as stated by felix. poppy, who is stealing away in private to meet with her fiancé, to the point of forgetting that she scheduled something with fifi. who is being given red roses by her future husband. penny says she isn't the jealous type, but after a lifetime of watching poppy get everything while she didn't... she certainly seems pretty jealous.
entering the realm of canon informed speculation here, I'd be willing to bet that penny saw marrying fitz - entering the fellow family - as a way to attaining the love she so desperately felt she deserved. she was closer with the fellows than her own family, seemed to want to be close with them. her dialogue related to flora, to freya, even to fifi, implies this to me. she wants to consider them friends, seems to admire fifi's intellect, wants to be close with flora who also likes birds, seemingly was already close with freya. freya was the only one who bothered to truly listen to her. I don't think she wanted a portion of the fellow inheritance. I think she wanted a family who loved her, who cared for her, who paid attention to her even slightly. it's a sad picture she paints. we know fitz wasn't the one who gave her that, unfortunately. he didn't remember the names of even her three most favorite birds. he remembers more about poppy than her, even. but marrying him, becoming a fellow... everyone else in the fellow family seems to hold her in decent regards, if nothing else. freya painted for her, gave her the gift she seemed to treasure most of any she'd ever gotten. fifi admired her, seemingly, judging by the way she talks about penny when you ask about her. felix doesn't seem to have any issue with her marrying into the family either.
of course... once freya found out what she'd done, she knew she couldn't have any of that after all. thus the events of tangle tower were set into motion. sure, she was snooping around for the beetle even before that, but that too was more recent. I do genuinely think whatever her end goal with the beetle was, it was something sort of last-ditch and desperate now that fitz - and thus her connection with the fellows - was sliding out of her grasp. freya's own discoveries only hastened things.