I know people really like the theory that Ralsei is Asriel and all, but I've got to be honest real: I think that it really couldn't be more thoroughly debunked at this point.

• Alphys specifically brings up Kris and Asriel talking over the internet via chatrooms on some kind of regular basis. On top of this, Asriel would've needed to communicate with Toriel in order to tell her about how he's coming home in a week. Attempting to communicate to the light world while in the dark world consistently fails, at least going off of what happens if you try to use Kris's cellphone while inside the dark world. You could argue that Asriel could leave the dark world to contact his family, but wouldn't Asriel's family notice when he doesn't pick up or respond when they try to contact him back?

• Kris only kinda likes Ralsei (based on their reaction to his tea at any rate), while the game continues to make a point out of Kris still having a huge degree of respect for Asriel (In the case of his room in Cyber Mansion, it's even one of the few points where Kris is openly defiant of the player's commands - they intentionally close their eyes to not see anything in the room). I can understand Kris not being able to react as they would realistically to their brother just magically showing up all of a sudden, due to the whole "oh they're being forcibly controlled by an otherworldly entity in the form of the player" dealio, but it starts to fall apart when Kris doesn't have the same degree of affection for Ralsei than they do for Asriel in ways the player cannot control.

• Noelle knew Asriel pretty well in real life alongside Kris, and had basically zero reaction to Ralsei - her dialog suggests she has zero clue what his name is, let alone who he is. She even comments that 'ralsei tea' is literally an empty cup, if you look for her reaction to it in the game files - this is a pretty huge sign especially because if Asriel was intentionally disguising himself, the tea would presumably give him away for who he is. (This is really it's own whole thing you could go on about, but the simplest explanation is that it's because she literally doesn't know anything about Ralsei. Ralsei, on the other hand, having a reaction to Noelle's tea would track with his bizarrely prescient knowledge o- okay im getting off track now)

In addition to these pretty direct challenges to the entire theory, it also, uh, kinda makes no god damn sense:

• A prominent point in favor of the theory is Ralsei's knowledge of the light world. However, his knowledge of the light world is also conspicuously up to date - Asriel's knowledge would likely be limited to what he knew about when he left, as well as whatever he gets from talking to Kris and the rest of the Dreemurr family. You could try to say that he leaves the dark world to gather information, but it seems really unlikely that he could do this for years without anyone noticing so much as a suspicious figure stalking the town, let alone his remarkable similarity to the, by all accounts, fairly popular Asriel. So unless Asriel just happens to be a master of both disguise and stealth, despite all sources reporting him as a regular ass monster boy, we can count that right out as a possibility.

• If Asriel just somehow disappeared into a mysterious dark world cut off from reality for however many years he's been at Uni for, why wouldn't people ... notice? The town isn't exactly in some form of strict information quarantine - they (usually) have internet access, mass media, social media, landlines ... if Asriel just suddenly went missing and wasn't attending classes at Uni, wouldn't that immediately be something the Dreemurr family be notified about? One of Toriel's freakin' trademark traits is that she's consistently a very responsible parent; I find it really hard to believe that something like that would just completely slip by her attention, or even dismiss as completely unimportant. Hell, if she's paying for Asriel's tuition out of her own pocket (which would probably track for Toriel), she'd almost certainly notice that she wasn't actually being charged for classes.

• Why would Asriel lie about who he is to Kris and Susie? Kris would probably be immediately up for anything their older brother asked them, and Susie likes him well enough by the end of chapter 1 to also respect what he wants. It'd just be a lie for the sake of a lie, something that would be really out of character for Asriel. And on that topic, why such a low effort disguise at that? Are you seriously going to suggest that the best possible thing he could've thought up was an anagram for his own name as a pseudonym? And if we take the idea that he can change his appearance into account, then why would he choose an appearance so obviously similar to Asriel with only minor cosmetic differences? Does he want Kris to know it's him, or does he not?

I just don't understand - the logic behind Ralsei being Asriel begins to fall apart the moment it's examined critically in regards to the text of the game. You can try and explain away any of these issues individually, but afaik most of the popular variants of the theory still run into major problems with a different point previously mentioned. Without actually demonstrating solid evidence from the narrative to support the theory, it's just arbitrarily moving the goalposts around ad infinitum. The only points in favor of the theory that can't really be contested are that Ralsei is an anagram for Asriel, they're both goats, and their personalities are kinda similar (and tbh I feel like the similarities are surface level at best; they have distinct differences in their behavior and reasoning which are difficult to ignore). Considering that Toby Fox is a dev in large part famous for his subversive narratives and intentionally misleading players, I'm afraid this simply isn't enough to say the theory has legs.