I agree with Chikaon on most points. Tsuzuki is the most level headed character. It makes her come across as cold/reserved, when Kashiwai is in the throes of passion, but I take it as Tsuzuki being more experienced when in comes to managing her relationships. While she's indeed attracted to Kashiway too, she sets clear lines and rules. Which is logical, since she's been managing an open relationship, with her at the center, for some time, while Kashiwai has only known standard monogamous relationships.
The fact that Tsuzuki mentions "the rules" shows that the author did her homework on real life polyamorous/open relationships and that she doesn't try to cast Tsuzuki in a bad light, but as someone operating on a different set of values. That people project aggression toward her is indicative of their own biases, like her highschool friend in the flashback.
I don't even know why we're talking harem here. While Tsuzuki seems to be the center in her relationship with Hiroki and Fukunaga, either of them could be in a relationship with other people and everyone is fine with that.
Tsuzuki is more in an "open relationship" than a polycule actually. And it's perfectly fine.