#Gah it's deliciously a bit complex and I just know Sonic is gonna be a bit dense about it. This is a situation where even if Sonic decides

[hadesknockedupintheunderworld]

Oh ho ho what a highly interesting question, anon👀

(Answered under the cut because it's a bit long)

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Before I give my proper answer, allow me to explain a bit of my interpretation of Tails. I actually see him as a sort of naturally jealous person with some innate possessive tendencies who has grown used to curbing these tendencies of his, which he had to do due to a number of factors.

One major reason is due to Sonic himself. To me Sonic has always had a complicated relationship with different kinds of love or attraction (and I do see him as on the aro/ace spectrum). So, when absolute strangers (or those he doesn't particularly like) obsessively pursue him for clear romance/possessive reasons, he tends to react with strong rejection or indifference, in contrast to how he welcomes ego stroking and praise from those who can be merely categorized as "fans" (or those he does particularly like). To me, Tails' initial admiration and pursuit of joining with and becoming like Sonic very well parallels how Amy Rose began to admire and pursue a romantic relationship with Sonic. Even if we consider that Sonic did save Tails that one time from being bullied (according to Sonic Prime and a new animation for Sonic 2 present in Sonic Origins), his pursuit of trying to work together with his hero (which is detailed a bit more in the jp manual for Sonic 2, and includes Tails going so far as to follow after Sonic everywhere and modify Sonic's plane) really isn't any more "normal" than how Amy begins to pursue a romantic relationship with her hero after Sonic saves her from Metal Sonic. The only real differences here were that Amy was explicitly pursuing romance, and that Sonic rejected Amy's advances but eventually took Tails on as his sidekick/little buddy.

This is also not to mention the implications here and there in what we can find of Sonic and Tails' backstory that Sonic would continue to run away and pretend he did not care, while secretly keeping an eye out for Tails.

And beyond that, as one of Sonic’s oldest friends, Tails gets to spend a lot of time around Sonic, and gets to see firsthand how he reacts to romantic prospects, his complicated relationship with "love" as a concept, and is present as Sonic’s group of friends steadily grows.

With all of that, I at least see Tails learning to deal with his innate jealousy and possessiveness so

Sonic doesn’t reject him

Because he doesn't want to keep Sonic from pursuing relationships with others

And for the sake of one's image/being a good person

If Tails has learned that confessing and making his romantic intentions clear could lead to Sonic rejecting him or it ruining their friendship (since he's seen first hand how Sonic has reacted in certain cases to love confessions or overt romantic affection), it makes sense to me that he would try to hold his feelings back (both those that are not squarely platonic and are explicitly romantic and/or sexual). And when it comes to Sonic, there's a guilt factor and an impossibility factor when it comes to keeping him from other people. Not only would Tails feel at least a little bad for restricting him, but Sonic does not respond well to being controlled. Now of course, how he reacts does depend on the time of his life and who it is, but there is nevertheless a futility in trying to control Sonic "I have no master except the wind that blows free" Hedgehog. And finally, my view on both Sonic and Tails, well... I believe that image and visual alignment is a big thing for these two, and that the two are secretly more...in the gray (or at least, that they're surprisingly immoral so to speak). In their origins to me, Tails is just a kid, minding his business, who does not inherently seek to hurt others (though his faith in those around him does matter), and Sonic is just a kid, free as the wind, who doesn't particularly care about being a good guy, but loves the attention. Essentially, though he does save some people on whim, the idea is that Sonic originally fights Robotnik simply because the Doctor, his machines, and his pollution have such a negative effect on his home. So beyond this point, he fights as a hero both out of interest of having his home intact/being alive, and because (simply put) he likes what comes from being considered a hero (all the praise and perks). And so because Sonic is someone that Tails admires, because Sonic has to uphold a baseline image or reputation so he can still be considered a hero/good guy, Tails becomes accustomed to doing the same. That's a bit of an oversimplification (how I see them and why is the subject of a proper essay about that), but the point is that Tails is also conscious that trying to overtly control or manipulate Sonic would not objectively be considered "hero or good guy behavior", especially if it goes too far.

And so with all of that plus the fact that Sonic has never seemingly accepted anyone's romantic inclinations or dated anyone, Tails curbs his natural inclination towards jealousy and possessiveness so he can be content enough to be at Sonic's side. Perhaps there are more friends Sonic is going to get, people he's going to show an interest in, but Tails can put stock in the fact that it's unlikely that Sonic will date anyone. Plus, even if that somehow does happen, Tails is largely fine as long as he's guaranteed to be by Sonic's side no matter what. So as long as he can be with Sonic forever, no matter their relationship, he's as content as can be.

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But then Sonic starts dating Nine, right?

And hahaha don't get me wrong here. Tails is pretty practiced. He's not gonna blow up or anything.

But once he gets past the initial shock of that being true, I think he just feels jealous and conflicted by default. I mean, he's just been accepting that there's no hope of ever being in that kind of relationship with Sonic. And not only has someone gone and done it, it's not just anyone.

It's a version of him.

And if a version of himself managed to do it (a version of himself who once lashed out at and fought against Sonic no less), then what does that mean about him? Does it mean that there's something about Nine Sonic likes better? Does it mean that he's had this entire situation wrong? Does it mean he lost his chance and he'd been so afraid for nothing? Does it mean that he could have had that this whole time?

And that's not to mention navigating this. Even if I believe Sonic is very very capable of having multiple best friends or even partners, it's only natural for Tails to get a little afraid of being replaced (especially if he learns that Sonic also considered/considers Nine as a best friend also)

But Tails is the type to not want to burden Sonic with all of this while becoming increasingly more worried. And although he would naturally have a complicated relationship with Nine, I still think he would generally like him and not want to take out his feelings on him. So with that being said, I think he'd sort of try to hold it in and act normal while really not acting all that normal, because though he could hide his feelings better if he wanted to, he still kind of lets it slip because he kind of does want to talk to Sonic about it or let his feelings be known.

I think that whenever Sonic eventually does see that Tails is acting a bit off he'll eventually talk to him about it or try to get him to, but, honestly, even if Sonic could say things (true things) that would make him feel better, I don't think Tails could be as content as he would be if it was idk... Knuckles or Shadow or what have you. Because if it was anyone else, Tails could be more content knowing that his relationship with Sonic is special and that he'll always be by his side, even if he's dating someone else.

But with the fact that the person that Sonic has chosen to date is another version him—a version of him which Sonic has also kind of clearly developed a similar "best friendship" with? I feel that even if Sonic tries to reassure him, Tails will forever be plagued with bitter and jealous feelings. Even if he'd never force Sonic to stop dating anyone, much less force him to date him, I don't think he can even really feel content here unless Sonic chooses to date him also if that makes any sense.

So, yes. That's my answer. Tails would try to be really good about it, but he's still bitter and annoyed and jealous and wondering if he's enough or has been doing something wrong. He's wondering why it couldn't have been him. And he won't be able to just be content at Sonic's side if he can't also be with Sonic. So while I can't say how long it will take, something will have to give eventually. I think at some point in their future, Sonic will have to tackle his specific feelings for Tails and they'll have to talk about their specific relationship, especially if he's dating Nine right in front of him.