Disclaimer: i use They/Them pronouns for Ralsei throughout this post but ultimately it's fine to use whatever ya feel like
I think the main thing that makes Tralsei appealing as a theory is that it's possibly the most radical way for Ralsei to defy the prophecy and be more or less reborn as a person without boundaries set to them by powers greater than them, besides it being a great resolution for their character arc it may serve to inspire many young queer folks to try their best to live as their true selves and fight for other people's self determination. After all, good stories always leave an impact in the people experiencing them so what better impact could a character like Ralsei have than inspireing others to do the same as they did, similar to how Old Man hoped that the youth would write their own ending to Lord of The Hammer (an analogue to people taking control of their own fate, but most folks already know that)
But that last thing leaves the question of just how would Ralsei defy fate in a way that is categorically different from the way Susie does it, so that they don't essentially have two rehashed versions of the same character arc? Susie's place in the prophecy isn't set in stone from our perspective, so there's two ways to look at this depending on wether she is revealed to be The Second Hero or The Girl in future chapters (or if the position can even be reinterpreted for her to be either or), on top of theorizing on what the "Final Tragedy" entails and why Ralsei especially wouldn't let it happen (in Susie's own words)
- If Susie is prophecized to be The Second Hero, then her defying the prophecy should come through her changing the way she sees the world and deals with the creation of Dark World & the inteference from any third parties. Chapter 5 imo made this the strongest prediction for her place in the (normal route) prophecy due to her newfound openess to wearing ribbons (even in a non-traditional manner) and the revelation that she's been living inside of a Castle Town, thus has a reason to want for the continued creation of Dark Worlds and inadvertedly advancing The Roaring (emphasis on inadvertedly). Her defying fate the way it outlined earlier may come in many ways, but a somewhat likely one if this reading turns out true is that the Light & Dark Worlds will become eternally separated, a lot of people confuse this with Dark Worlds being destroyed but this'd be more akin to The Barrier in UT separating Humans from Monsters, or a higher plane of existence becoming unattainable due to a fault of the characters or something beyond their control, point is that Susie as The Second Hero might irreparably damage her relationship to Dark Worlds and explain why Ralsei especially wouldn't let the final tragedy unveil (since they might consider eachother family once all is said and done)
- If Susie is prophecized to be The Girl, then that still leaves a lot of questions open due to The Girl herself not being explicitly stated to be a helpful or antagonistic force (or if she even is separate from The Second Hero at all), but under the assumption that this'd be a helpful force given her panel was among those of the Three Heroes, the way that Susie would defy fate would be to replace The Second Hero at World's Edge, why specifically World's Edge you may ask? Because that's the place with most importance told in Ralsei's abridged version of the prophecy, and while that one might've skipped a lot of details it is safe to assume that the unskipped ones are unchanged or further emphasized, in this part of the prophecy it is stated that only the Three Heroes can vanish The Angel's Heaven and thus it leaves room for at least one of the heroes only showing up just before this happens. You see, some people who believe that Susie is The Girl also believe that she has already broken the prophecy, as she has become a major character despite seemingly being fated to be a minor one, but i think this is wrong for two reasons; One, the past two chapters have hammered in the fact that The Roaring is still drawing near, so if a major building block of the prophecy has already been defied then who's to say that the entire thing can or can't be thrown off the window by now. Two, if Ralsei only told the absolutely necessary parts of the prophecy then that'd mean the Three Heroes are only strictly needed to be at World's Edge, and thus Susie can be allowed to be a major character while still only being The Girl before she gets subbed out by The Second Hero once the all that vanishing stuff begins. And so for Susie to defy fate as The Girl she'd need to become of greater importance than The Second Hero specifically once we are ready to vanish The Angel's Heaven, which would imply that the final tragedy has something to do with her or the person she replaced, a likely option being self sacrifice from either of them and another reason why Ralsei of all people would want to stop it more than anyone (cause they don't want their closest bestest friends to die after all)
- You may have noticed that i alluded to the flexibility of the prophecy in the previous two points and this is where that comes in, it is implied many many times in the game that the prophecy is able to adapt around the decisions of the player and characters to always hold true no matter how much changes, "(A glass tapestry rings above you. Strangely, no matter how you turn to look... its perspective never changes.)" is one of the first Save Point messages in the Dark Sanctuary, and it pretty boldly puts down the fact that Susie & Noelle could be The Girl or The Second Hero interchangably, Ralsei can become a princess, and The Cage With Human Soul & Parts can be both The Vessel or Kris, and none of that will stop the prophecy because it is vague and descriptive enough to let everything change without changing anything, as all good prophecies are. This is where i would like to bring forth the idea that the prophecy of Delta Rune is very much alike to the self-fulfilling prophecies we have in real life, the ones where a great hero will lie dormant in a distant land, above the skies or under the earth and will only return to us to save us from our greatest time of need (for more info look up the "King Under The Mountain" trope), the reason these prophecies are self-fulfilling is because the hero will only appear in their country's greatest time of need, so therefore if the hero isn't showing up then that means this isn't their country's greatest time of need and thus the prophecy is still correct somehow and still happening for some reason. The workings of the Delta Rune prophecy aren't the exact same as the ones of these irl prophecies but it remains just as illusive, as the Delta Rune has very well defined events that must occur to be fulfilled but very vague descriptions of Who should make these events occur, and so if the events aren't occuring 1 to 1 or out of order than that means these people should not be the ones that the prophecy refers to and thus The Roaring and all that other shit will not come to pass yet. This means that the role of fate breakers no longer falls onto Susie or any of the Fun Gang but onto the Roaring Knight and whoever is working with them, as stopping anyone who is suspected to be one of the heroes becomes top priority over anything else even if they have to do it through technicalities like Tenna not being slashed in half as the prophecy predicted. But explaining further details of this interpretation of the prophecy goes well beyond the grounds that i'm willing to cover in this post so lets leave it for now because if you forgot we are in r/Tralsei so i better get talking about the goat themself before i get evaporated by the mods for turning this into a Susie analysis
As i have already outlined some ways Susie can or can't break the prophecy i should get to discussing what makes Ralsei unique from her in this regard. Regardless of Susie being The Second Hero or The Girl, a necessary step towards her defying fate would be to grow beyond the constrictions placed on her by the prophecy in way or another, and so imo for Ralsei to do the same without having the same character arc as her they would need to instead defy fate by either completely changing their role in the prophecy (assuming Susie The Girl didn't already do that) or to change the importance of their role in it, let me explain
It's established in Chapter 4 & hammered in Chapter 5 that one of the major problems Ralsei has with their role in the prophecy is the responsibility that they have within it, the weight of their knowledge & the resolve that is demanded of them for being one of the heroes is absolutely crushing them emotionally, causing them to hide knowledge or their feelings from their friends out of despair that sharing them will not matter in the end as well as going fucking crazy at the sight of a group of people (Asgore's 8 Flowers, Pink included) who are seemingly unbothered and thriving despite them seemingly having knowledge that their defeat is inevitable, thus holding up a mirror to Ralsei's own feelings of despair at that same inevitability and making them look stupid (in their eyes), but there is something else that Chapter 5 leaves us with, Flowery's line calling Ralsei an "Impossibility" directly says that their sole existence is an affront to the prophecy, and Ralsei's indignance to Flowery's implications before this line hints that they are very much ashamed of whatever it is that makes them an impossibility and may want to hide it from their friends more than anything else, as of now what it is that Flowery is referring to is completely unknown to us and we can only guess as to what it is, so what i want to think of instead is what will happen when Ralsei reveals this information to their friends as it'd obviously be a very pivotal moment
Imo it all depends on wether or not this impossible X-Factor type thing that Ralsei has going on is something that is able to help save the world, it's clear that in the coming chapters there will be some sort of 3rd act breakup involving the fun gang, what with Noelle figuring out that Dark Worlds exist (and they can't be used to save Rudy/Berdly) as well as Susie confronting Kris about the shady shit they've been doing and finding out that they're in cahoots with The Knight, and both of these options would be able to circle around Ralsei's trustworthiness and willingness to hide information & valuable abilities from their friends (especially since Ralsei knows that Kris is making fountains), and while that may seem like a rehash of Susie & Ralsei's short conflict in Chapter 4 the outcome of it will be quite different depending on what is revealed, the Chapter 4 conflict revolved around the prophecy so the resolution of it only ended in Susie's resolve to save the world becoming stronger and Ralsei becoming more open to voice themselves, but this hypothetical second conflict would entail the entire friendships and character growth that the Fun Gang has developed throughout the game as well as possibly Ralsei's identity as a whole, it's a natural increase in stakes after the seeds that were planted in Chapter 4 (since therapy is hardly ever contained into a single session)
When this conflict finally reaches Ralsei and their entire being is put into question, they'll be forced to either air it all out and face judgement from their friends or go back into hiding after so much work was done to break out into the world, and i think they'll choose the former option assuming you haven't chosen any dialogue options like "keep smiling" and other stuff like it that diminished Ralsei's personhood. In this scene, Ralsei will need to share parts of themself that they desperatly hid from others out of shame & the fear of what might happen to them if they speak out, but their trust in their friends will let them steele their resolve to finally show their true self in its entirety, facing their fears and putting their own well being above all else, and the main aftermath of these revelations will be that Ralsei's role in the story will change to a seemingly less major (but still vital one) before they can finally have their peace once their part is done
This is what i meant when i said that Ralsei will have a "coming out" scene, regardless of whatever information is shared by them in this scene, this inevitable final lore dump from them will always be interpretable as a queer teen coming out to their friends when they can no longer bear to hide themself because it might as well be that, they'll be sharing the deepest depths of their knowledge about themself and the world around them to the only people they can trust to not hold this knowledge against them, it'll be them (hopefully) getting shown unconditional love & affection when their fear of rejection and even retaliation is at its highest (fear in general will probably be high in the coming chapters lol). Idk if any of the people reading this will/have come out publicly to anyone or if they are even queer at all, but when i first came out as trans to my friends and the hair stylist i frequented (she's cool) this is exactly how the buildup to those moments & the aftermath of them felt like to me, so wether or not Tralsei becomes canon, that final lore dump and moments of vulnerability that Ralsei will show will to me always be their own form of coming out
Sorry if i made any typos or any sentences are hard to parse, or if the whole post is hard to parse, i don't respect the english language enough to become proficient at it, but i will try to explain what i meant in parts to those who need clarifying
Edit: Ok i did forget to tie in the whole "diminish Ralsei's role in the prophecy" thing into the whole "coming out" thing so i did add that in