Thank you for reading this story!
This final "chapter" is not so much a chapter as it is simply a place to talk about some of my headcanon that went into making the story.
Chara's character
Most of the early fandom portrayed Chara as a murderous psychopath. Fortunately, that has given way to a more nuanced view of the character. Fan art, however, hasn't fully caught up. It's not uncommon to still see kindhearted Frisk with a stick standing next to a grinning, demon-eyed Chara holding a blood-red knife. In contrast to that, my stories mostly follow the Chara-as-narrator model. Chara is a girl somewhere between 10-12 who has been emotionally scarred in the past and harbors deep anger towards people of all kinds. (How deep, or how long she holds to that universal hatred, is another question.) But after her spirit is attached to Frisk throughout the events of Undertale, she learns from his kindness, compassion, and morality and begins to heal emotionally. By the time she is resurrected (and even more so by the time we get to the events of Better Than New, the next story in this series), she is in a much better place mentally.
Chara's mindset
A reader asked some very good questions: why would Chara conceive of her plan in the first place? And why in the world would she think that the Dreemurrs (Asriel in particular) would be willing to accept her death in exchange for their freedom? It's clear to the reader how misguided Chara is, but unfortunately she doesn't see it that way, for several reasons.
First, although Chara is quite bright, she's still only a child. She's told herself a story and is enamored with the happy ending it will bring without fully thinking through the consequences.
Second, and far more seriously, Chara has never been with people who really, truly wanted her. She has only ever experienced outright rejection, or family situations that allowed her to think that maybe she had found a true home, only to have it ripped away from her. Her defenses and emotional walls are fully in place now. She knows the Dreemurrs have been very kind to her, but she is sure it's only a matter of time before they grow tired of her, like everyone always has. She won't allow herself to hope that the happy times she has with them are permanent. Sooner or later, they'll want to be rid of her.
(This is no doubt compounded by the fact that, canonically in the game, Chara accidentally poisoned Asgore. Though they forgave her, it only reinforced her belief that she didn't belong with them, and they wouldn't put up her for long.)
But she can't help but love them, Asriel especially. Which is where the third factor comes in: Chara knows she has the key to their freedom. It's clear from the plaques in Waterfall, the longing whispers from the Echo flowers, and even the wistful things her adopted parents and brother say that their greatest dream is to one day leave the underground... and Chara knows she's the only one who can make it happen. She's quite certain that they wouldn't ask her to sacrifice herself (Asriel certainly wouldn't, and she's almost certain neither Toriel or Asgore would either), but she's also convinced that if she does, then in a year or so she will just be a distant memory to them. Maybe even a hero and martyr in their eyes! Better that than to be abandoned once again, which she's sure is inevitable.
All those mistaken premises lead to a decision that seems entirely logical to her - noble, even.
Asriel's role in Chara's plan
It seems very strange in the Undertale storyline that Asriel would participate in Chara's plan to die, even going so far as to help fetch the buttercups. Why would he do that? He clearly loves his sibling – how could Chara possibly convince him to go through with the plan? I am unwilling to accept that Asriel would willingly allow Chara to die, even if it broke the barrier and freed all the monsters. No matter how much Chara demanded, pleaded, cajoled, or mocked him, he would still tearfully refuse. Asriel may be tender-hearted, but he's not that big of a pushover.
The only way he would go through with it is if he didn't think that Chara would actually die. And given his trusting nature, combined with the great deal of ignorance monsters had about humans, Chara was able to sell her whopper of a lie to Asriel. She told him her "death" wouldn't be permanent. Ordinarily it would be (she told him), since her poisoned body would be unable to support her soul, but as a boss Monster, Asriel would be in a unique position to absorb her soul and keep it safe. Once they were through the barrier, she assured him, Asriel would be able to remove the poison within her body, then restore her soul. Convinced by the lie, Asriel was finally willing to go through with it.
Asriel's transformation after absorbing Chara's soul
Another common theme in fan art is to assume that, once he absorbed Chara's soul, Asriel took on the same adult form that we see during his fight with Frisk. But that's not necessarily true. It took the power of six human souls plus the souls of almost every monster for Flowey to achieve that form. I think it's more likely that Asriel+Chara took on the appearance described by the plaques in Waterfall: "a strange creature", "a horrible beast with unfathomable power," something "very unsettling." In addition, although it does not excuse the human villagers for what they did, it is more understandable that fear might cause them to attack a monster that looked truly terrifying, rather than a noble monster prince wearing royal robes, as he is usually depicted when he carries Chara's body to the village.
Flowey's character post-SAVE
What was Flowey like after he transformed into Asriel, was SAVED by Frisk, said his goodbyes, and transformed back into Flowey? Did he go back to being the same compassionless Flowey he was before? I don't think so. At the end of the Pacifist route, Flowey doesn't even want the power of resetting any more. He's happy that monsters have returned to the surface, believing that "peace and prosperity will rule across the land". He practically begs you to not reset, so that Frisk can live a happy life. It's clear that Flowey is not the same as he was. In this story, I assume that the effect of the SAVE on Flowey is permanent. His form reverts from Asriel to Flowey once he gives up the souls, but his compassion and morality are permanently restored.