Imagine the Tails Polycule realizing thier feelings for each other.
Ooooooh
I feel like this one is a bit related to the ask I answered about their first kisses, which I'll link here
But (while I can't promise any specific events discussed here will happen the same by the time I get around to fully fleshing this au out) I'll answer this to the best of my ability as I understand them right now
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Tails/Nine
This already sets up a fairly large hurdle for Tails and Nine's relationship, which requires getting to know each other better (reconciling who the other actually is with the kind of fox they expected), the opening of hearts, and for each to deal with any negative feelings towards each other.
So, as I said in the kissing post, I can only see Tails and Nine having a slow burn relationship. Tails is the catalyst for Nine's identity issues and a reason for insecurity in his friendship with Sonic. I think that with Nine also, he built up an idea/image in his head of Tails based on a combination Sonic's stories and the idea of fox he (Nine) personally isn't like that he believes Sonic expects from Tails and is naturally expects out of Nine (a take built off of negative feelings by default, likely around S2 era). As for Tails, he likely gleaned what he could about Nine from Sonic's stories and his first impressions.
However, on top with having to surmount the hurdle of negative feelings and learning to open themselves up to each other, the (potentially even bigger) hurdle is Nine's lack of understanding when it comes to what love is.
I don't know if I've said it here before, but I personally read Nine to be on the aroace spectrum. I think that before he met Sonic (and eventually became friends with him), he didn't recognize that he was lonely. He'd always thought that he was better off without anyone else and that letting anyone in would be opening yourself up to betrayal (after all, he didn't believe in the inherent good in people anymore), but being with Sonic opened him up to feeling things he had never felt before.
Positive feelings towards someone else, a sense of warmth when being cared for by this person, being the center of that person's attention, feeling a real sense of hope for the future, experiencing friendship, the feeling of working towards shared goals, the wish to live your future with that one person beside you. And because Sonic's presence came to fight the loneliness deep in Nine that he himself hadn't acknowledged before, when Sonic was gone and Nine became alone again (after Nine gets captured in Episode 6, post the S2 finale, and post the s3 finale), the loneliness started to feel more like an open wound. After becoming friends/best friends with Sonic, I believe Nine's own loneliness became impossible for him to ignore (which perhaps was even more lonely now that he'd had a real taste of being around/with someone who made him happy).
To be clear, Nine was feeling a lot of things for the first time. And I don't even particularly believe he'd finally come to consider those feelings for Sonic as any specific type of love by the end of Prime. I just think that he'd come to accept that he liked being Sonic's best friend, that Sonic made him feel things, and that what he really wanted is for Sonic to be at his side.
So this also means that...I think Nine realizing his feelings for Tails would have to happen very slowly. While the realization moment would likely be Nine realizing that he's become attached to Tails too and cares about him, it would probably take some help and self reflection to actually sus out the exact nature of his feelings (feelings I think he will never be able to easily categorize as romantic or platonic).
For Tails, I think he has better grasp on romance, friendship, and love than Nine does. To contrast Nine, his realization of feelings moment would not be when he realizes he cares about Nine, but I feel like it would either come when he realizes he's begun to factor Nine's existence into his future as being someone he wants near him, or when it hits him just how much he considers Nine's future happiness, how good it feels when Nine is happy.
I think Nine would be in denial for a while due to the many reasons already stated. And I think Tails would be in subconscious denial until either someone calls him out for how he acts around Nine/treats him, or until Tails starts to compare his feelings for Nine and Sonic (or maybe they even happen in that order😂)
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Tails/Sails
As I went over in the first kiss ask, more than likely (especially with them spending more time together than they usually might due to the mentorship) they'd end up grappling with the confusing nature of their feelings over time. So, rather than a solid realization moment, I feel as if these two would have some feelings curiosity for a while. So their "moment" would be less based on the moment they discover their feelings and more focused on the moment they confirm the nature of their feelings (or at the very least confirm that they do have some more romance aligned feelings among the jumble).
A kiss is a pretty decent way to attempt to feel out one's own feelings, and (in cases like these) both enjoying it can potentially be a confirmation on its own. So...yeah. the setup from the first kiss ask. Somehow the two end up agreeing to try a kiss "for scientific purposes" as an excuse to guage their own feelings and the other's feelings, and when the two clearly like it, well...the experiment has served its purpose.
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Tails/Mangey
Starting with Mangey, I think (comparative to Tails) his realization will come fairly quick and easy.
Now, since Mangey starts out on pretty good terms with Tails (triple threat combo of having no reason to dislike Tails, Tails making a good first impression with Mangey, and Sonic having good things to say about Tails), all I feel that's required for him to develop specific feelings for Tails is for him to get closer to/learn more about Tails, and for Tails to prove himself trustworthy. I mentioned during the first kiss ask that Mangey would probably be curious about Tails and want to learn more about him from the getgo (in this case, because Tails seems interesting, because Tails is another version of himself and that's interesting, and because Sonic seems to like him). This setup makes it so that even before Tails himself really gets to know Mangey, it's possible for Mangey to learn more about Tails. On top of that, it's fairly easy for Tails to prove himself trustworthy in this case. All he really has to do is convey to Mangey that he (Tails) cares about him (Mangey) (stuff like Tails rescuing him or helping him out during a mission, giving him a snack/treat, etc).
Honestly, with that being said, I don't think Mangey generally has big realization of feelings moments with anyone. I feel as if his feelings kind or shift and grow over time. Then, when that moment of realization does come, it's more of a moment of acceptance. In other words, he just sort of realizes/notices that he's started wanting this person to be around him forever, or notices the urges he had where there were none before, or that it's comforting to be around this person now and kind of rolls with the punches on it. In a "Oh, so this is our relationship now" kind of way. His relationship with Tails is no exception here.
And by the time Mangey's feelings for Tails have developed fully, I think he'll have already come to terms with the fact that his feelings have changed a little (that, well, their relationship gotten stronger and he's gotten closer to Tails so it's only natural he'd want to spend time around him and show him neat things and he feels like kissing him sometimes), as opposed to having some big realization moment.
This is also one of the few cases among the polycule members where I don't think a first kiss will cause one of them to realize or confirm the nature of their own feelings. With the Tails/Mangey first kiss in particular (as detailed in the first kiss ask), that is a moment I feel could allow Tails to recognize the nature of Mangey's feelings for him (assuming Tails doesn't just assume that Mangey isn't aware of the meaning of the gesture). Mangey would have already accepted his feelings by that point. However, as for Tails' own feelings, I think the important part of that scene for him (rather than specifically the kiss) is how it functions as a bonding moment between him and Mangey. It allows the two of them (from Tails' perspective) to overcome some obstacles and grow closer to each other (which also naturally allows Tails to improve his idea of who Mangey is and deal with any lingering jealousy better).
As for Tails, I think his realization of feelings moment will take quite a bit longer to hit, and I think part of the reason for that has to do with that bit of jealousy he has over Mangey that I mentioned in the first kiss ask. I feel as if Tails will have to get closer to Mangey and get to know him better for feelings to grow, and dealing with his bit of jealousy and incorrect first impressions of Mangey is naturally entwined with that.
With that also being said (kind of like with the Tails/Nine answer from earlier), I feel as if Tails will have to eventually start recognizing the way his feelings are becoming muddled/changing a bit and compare his feelings for Mangey with others he has feelings for to sus out the nature of them.
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Nine/Sails
I kind of touched on it in the first kiss post, but I believe that Nine wouldn't be able to recognize his feelings for anyone else until he (at baseline) deals with his hangups on Sonic and Tails.
To clarify, I think he's the type who suffers from a phenomenon where he can genuinely have some type of thing going on with others and harbor secret feelings for them, but isn't able to recognize what's going on (and in some cases fully feel those feelings) as long as these relationships are overshadowed by the ones he's hung up on.
As for Sails (like all versions of Tails in my readings of them), I think he's a fox who cares quite a bit about actions ("actions speak louder than words"). So, especially after S3, I think Nine would need to consciously make amends with him, not backstab him, and prioritize the well-being of him and others for Sails to be able to fully trust him. It's at that point I think Sails' feelings would be able to grow, especially if he and Nine start hanging out together and getting to know each other better. With that, I'm not sure how sure Sails would be in his own feelings. Rather than having a solid realization of his feelings, he seems the type to me to go through interactions, which then cause the puzzle pieces to click into place, until he eventually has enough of an idea of his feelings to take a leap of faith on them. So when he realizes his feelings for Nine, it'll be after that buildup of beginning to feel "some kinda way" over simple things like Nine rambling or smiling or getting annoyed, and it'll be after realizing he likes spending time with Nine.
Now, with that being said, I feel like Nine's default setting right now when he realizes his feelings/realizes he cares about someone deeply is a "Seriously? I'm attached to this person? Really?" (even when he doesn't dislike that person in any way). I also think that Nine is just the type where his feelings build under the surface for a while until sort of suddenly surfacing and becoming an issue for him. So, post actively dealing with his hangups over Sonic and Tails and allowing himself to open up to other people, he'd be on the path to allowing his feelings for someone like Sails to finally surface and no longer be overshadowed. In fact, it's very possible for that scene I painted in the first kiss post between Sails and Nine to be the moment where Nine realizes he has feelings and attachment towards Sails (otherwise, why would he like and reciprocate the kiss?)
Maybe one random day Sails looks at Nine and it squeezes his heart. He doesn't 100% know what it means. He doesn't wonder "why him?" He just takes note that he's feeling something simmering in him just under the surface, and he takes note of the proof that Nine does care about him (even if Sails does annoy him sometimes). So, when the time is right, Sails follows his heart. Maybe that moment of solidifying what he's feeling for Nine is like that moment from the first kiss ask, when Sails takes a leap of faith and goes to kiss Nine (giving Nine many chances to back out), only for Nine to reciprocate.
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Nine/Mangey
As with Sails/Nine, it would definitely have to be post Nine dealing with his hangups and actually allowing the walls around his heart to come down.
Mangey needs to be able to warm up to Nine and trust him, which requires Nine actively helping and showing that he means no harm to him or the other variants. But I think once things get to the point that Mangey no longer registers Nine as a threat, his feelings just kind of grow naturally. Instead of having a realization of feelings moment, he most likely becomes clingier to Nine than before as time goes on, and he would look for excuses to spend time around Nine. I feel that when his feelings realization does come, it's a bit like...quietly flipping on a lightswitch that's set on a pretty dim setting. So there's fundamentally a shift in his relationship with Nine and how he sees him, because Mangey's realized he wants to keep Nine in his life going forward, but that he's changed a bit suddenly is more apparent than why it happened. If that makes any sense.
As for Nine, I feel like it'll take a little while for him to warm up to Mangey in general. Before I even factor in him dealing with his hangups, one hurdle is bridging the gap between Nine getting used to Mangey's presence around him/getting used to hanging out with Mangey, and Nine having a realization that he specifically enjoys having Mangey around and would like him to continue to be around in the future. I feel as if this was easier for Nine to do with Sonic during the events of Prime because he was specifically seeking out a home and Sonic (the only person who'd ever cared about him as Sonic had at that time) was someone who felt...special. What I'm saying is that the events in Prime (including Nine considering what he wants for a future) lined up just so that he could bridge that gap and realize that he wants Sonic around (even if he potentially had to twist some things around to justify it being more/other than his personal want to do so), which Nine doesn't really have the benefit of in the post canon scenario the prime polycule au takes place in.
With that being said, at this time I think that Nine's "realization of feelings moment" will be a bit sudden like it was with Sails. I know this is probably cheating at this point, but I feel like the Ninegey first kiss scenario from the linked post is a good moment for Nine's realization to happen. By this point, Mangey would have already accepted that he likes Nine and wants to be around him, but, for Nine, a moment such as kissing Mangey on the mouth would be a bigger push towards him grappling with actually having feelings (especially if he's been pushing down inklings of realization for a while).
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Sails/Mangey
I don't think there ever really needs to be a distinct realization of feelings moment between Sails and Mangey. They bonded and clicked quickly and easily, and so I see them taking things like being clingy/around each other, caring for each other, prioritizing each other's well-being, and wanting a future with each other (like Black Rose/Rusty Rose) to be a given for their relationship. They'd only get closer as time goes on and so I see them taking these changes in stride, not really thinking too hard about the nature of their relationship and feelings. They're the type I can see getting together and becoming casually intimate (even in traditionally romantic ways) with each other without doing self reflection.
Of course, the idea that they never end up thinking on the nature of their relationship isn't fully realistic (as there are other people in this world), so it would probably be outsider povs as they talk to Mangey/Sails about their relationship that has them consider the nature of the thing. But, even then, them loving each other would be a given, so I don't think parsing out some of the nature of that love would change their relationship much.
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Thank you so much for the ask, anon! 🥰
I apologize for how long it took me to get around to it, but I did find working on this post to be a fun exercise in getting into these characters' heads a little. And, like always, if you (or anyone else) would like me to elaborate on any of my points, has further questions on the prime polycule, or anything else you'd like to know, feel free to shoot me another ask!!