If there's no war, do you think Cassie's and Jake's relationship could work? Tobias' and Rachel's?
My gut says that without the influence of aliens, Cassie and Jake would be fine—better, even, than they are in canon—while Rachel and Tobias would have never happened in the first place and wouldn’t have stayed together even if they had tried dating.
Their inciting incident, so to speak, is simply that Rachel notices their mutual crush and decides to get her inner Emma Woodhouse on. She’s the one who decides that they should all walk home together despite the sexism of it all specifically because she can tell Cassie’s interested in Jake (#1), she pretty much announces to Cassie that her interest is mutual (#9) and she goes through various Rachel-esque schemes to dress Cassie up nice and send her off to go hang out with Jake (#14, #24), until eventually she escalates to instructing Jake about when and where he will ask Cassie on dates (#29, #36). All of that would not only happen without the war, it would probably progress a lot faster without the war as this huge distraction taking up all their spare time. Jake and Cassie actually spend Thanksgiving together (#4) and regularly sit next to each other on the bus holding hands (#6) well before the war begins, so they’re headed in that direction before the war gets in the way.
During the course of their relationship, Jake and Cassie demonstrate a very healthy ability to discuss their disagreements in a way that does occasionally descend into arguing but never involves any kind of passive aggression or aggressive-aggression, instead openly airing their points of view and hashing it out (#9, #16, #31, #41). If they got married the way that Jake so clearly wants them to as of #53, I could definitely see them having a healthy long-term relationship: Cassie’s the more intelligent and dedicated one, so she’d probably be the breadwinner, while Jake’s better at leadership and general pragmatism, so he’d probably focus more on taking care of any kids or pets or houses they might acquire along the way.
The dissolution of their relationship, as unpleasant as it is, can also be traced directly back to the war. The specific incident in this case is of course Cassie preventing Jake from killing Tom, and that would obviously not be an issue without yeerks (#50). It’s worth noting that that specific incident also comes after a long decline in the quality of their relationship—in #41 Jake shows dawning awareness that they are growing apart, Cassie spends half of #44 flirting with some other boy while barely thinking of Jake, and in #48 the two of them have an argument that never gets fully resolved. However, so much of that dissolution comes from their mutual and growing anhedonia, which is unquestionably a side-effect of some combination of PTSD and depression that they both show as early as #16, that I am reasonably confident in saying that the war destroyed their relationship both in the acute and the chronic sense.
Although Rachel and Tobias would have met each other without the war, it’s not clear that they would have necessarily dated. Their inciting incident as a couple is the discovery in #1 that they are the most passionate about getting involved in the war and avenging Elfangor; if there are no aliens then they obviously don’t have that common ground. The process of them getting closer as a couple is at least partially driven by the fact that Tobias wants human company and Rachel’s his best option (he can’t stand the thought of either Jake or Cassie pitying him, while he and Marco just straight-up dislike each other at first). Most of what they have in common comes from the war: Tobias draws attention to the fact that they both have these dual identities (#32), while Rachel feels like Tobias is the only one she can talk to about her own willingness to kill (#17). These two do have more than enough common ground, but it’s all common ground built on a foundation of violence and predation.
Note as well that they’re not together if they’re not both Animorphs: MM4 has Rachel/Marco in that AU, while MM3 has Tobias/Melissa in the AU opening. Whereas Jake and Cassie are still together in both books. They’re not a happy couple (Jake’s ragingly racist in the MM3 AU, and dead in the MM4 one) but they’re together. Unlike Rachel and Tobias.
Rachel, like Cassie, toys with the idea of dating elsewhere, but a lot of what drives her away from (the unfortunately named) T.T. in #27 is not so much the realization that she prefers Tobias as the realization that she no longer has anything in common with the perfectly ordinary T.T. She and Tobias spend all their time fighting side by side, so of course they have a fire-forged friendship. These two also do not handle conflict well: in #37 Rachel casts Tobias questioning one of her decisions as “betrayal” and Tobias is passive-aggressive the whole time, in #13 Tobias literally gets to the verge of starving to death rather than have a conversation with Rachel about how he envies her human life, and in #33 Rachel arguably risks Tobias ending up trapped in morph just to have a conversation. Most of their disagreements go unresolved or set aside as something more important and yeerk-shaped comes up, and although they love each other passionately, they do not do nearly enough of the hard but necessary work of relationship-building. Jake and Cassie argue more, but Rachel and Tobias do a lot more “What’s wrong?” “Nothing.” “Then why do you seem upset?” “I’m fine. God!” etcetera.
Assuming there was no war, Rachel’s and Tobias’s relative places in the social hierarchy of their school would also become unavoidably more important and thus more of an obstacle. Rachel’s at the top of their social pyramid; Tobias is at the bottom. Rachel works to stay there, and enjoys her popularity; Tobias struggles to make friends and has resolved himself to perpetual New Kid-dom. (Contrast, again, Jake and Cassie: they’re both cheerfully clueless about the social hierarchy, and the only reason Jake ranks higher than Cassie is that he’s a wealthy gender-conforming white male jock, while she is none of those things.) Rachel and Tobias would almost certainly face negative social pressure if they dated openly, and would run into a whole host of other obstacles in a yeerkless world: they are both fiercely independent and prone to anger, they are both okay-to-bad at communication and reading other people, and they are both willing to fight really really dirty when cornered (#33, #22).
The evolution of Rachel’s and Tobias’s relationship is also not necessarily one of them becoming closer as it is them becoming more codependent as they both become more volatile. I have expressed my displeasure with Cassie’s assumption that Rachel is “the only person who ever loved Tobias,” but it also makes sense that Cassie, and Tobias, would see it that way (#54). Not only does Tobias not know what to do with himself without Rachel both in the short term (MM3, #29) and in the long term (#54), but Rachel puts some deeply unfair emotional weight on Tobias: part of the reason that she is so harsh to him in #32 and #37 is that she feels she needs him to look at her and see a good person in order to retain her sense of selfhood. Those two keep each other sane during the war. I think that if Rachel had lived they’d be capable of a healthy relationship after the war. But without the war, I don’t think they’d go beyond giving each other the time of day, and that they might be better off that way. Poetry might have put it best: their relationship is fun to read about, fun to write about, and flawed in reality.
