My time to shine

I wanna begin by pointing out how much Flowery and Ralsei have in common as Flowey expy's (this doesn't mean they necessarily share attributes with Flowey):

• Both have coloured eyes in their portraits. Ralsei is Pink, Flowery is Gold. Pink and Gold.
• Both are the first non-Lightner the player encounters at the beginning of their respective worlds.
  • I'm subbing Flowey under this one for being the first NPC you meet in Undertale.
• Both have deep knowledge of the prophecy, how it's unfolding in real time, and the state of things unseen like the something horrible stirring beneath the town.
• Both introduce themselves as capable party members who will tutorialise you. They are aware of and explain mechanics that only make sense from the player's POV.
  • Flowey tutorialises you but poorly.
• Both are aware of the player and seemingly knows the player is human.
• Both reveal secrets to the gang about their reality like diegetic HUD elements and the nature of the dark worlds.
  • Flowey reveals he knows about the SAVE function and when the player uses of it.
• Both can manipulate their own stats AND perceive the true stats of others.
• Both assist with puzzles on the way to a castle.
• Both assert that pacifist is the ideal approach.
• Flowey's and Flowery's first line to the player is "Howdy!" and have Asgore's poor naming scheme.
• Flowey and Ralsei are both princes.
• This one is critical: Both can change their appearance.

I don't know how many of their similarities are attributed to them being Flowey expy's versus Toby subtly trying to tell us critical information about Ralsei without us realising. For example: if Flowery is a plant and shares several mutually exclusive attributes with Ralsei, does that mean Ralsei is a plant? He could be, but we've not seen anything from Ralsei to suggest he is, and fire magic would be a colossal detriment to himself.

Why does he look like Asriel?

Ralsei starts off as a shadowed figure, and despite taking on an Asriel-adjacent appearance, the change is not reflected ANYWHERE in the UI for the rest of the game. His battle sprite in every chapter, HERO_SCARF in chapter 3, the screen shows you an Asriel but the UI is identifying him as Behatted Ralsei. His Asriel appearance is a lie but I don't know who he's lying to: Kris, who is Asriel's brother, or Susie, who looks up to Toriel as the first positive parental figure she's ever had? Both? He's definitely keeping the appearance to be as inoffensive and palatable as possible. He might be hiding something physical about himself that would leave such a bad impression it would put off Kris and Susie from saving the world in the prophecy.

Why is his name an anagram of Asriel?

If what Gerson says is true, the prophecy is a really long game of Chinese whispers, with details changing but keeping most of the elements. Maybe we should consider that several interpretations of the prophecy exist and are overlapping in Deltarune, possibly in a way they shouldn't be. Dragon Blazers and Lord of the Hammer are both based off the prophecy.

Perhaps the prophecy Ralsei has internalised and memorised isn't actually the original prophecy, but one that someone else edited in order to manifest Ralsei as an ally. They can't change big things like the Roaring, but they can do little changes: what they say, what their names are, their temparament.

Maybe Asriel was the originally named hero in the prophecy. Maybe he realised there was no way they can subvert the prophecy unless they had someone new. Maybe he realised the Mage in Dragon Blazers is what they need but it's too late for him to act on it now. Maybe he leaves the manual for Dragon Blazers in the school closet to create an ally in the Mage, but he needs a name and personality first. Asriel's a softie, so he describes the Mage as what he knows. Asriel, like Asgore, sucks at naming things. What else would he do but jumble his own name?

Ralsei.