I kind of want to write a post about Undertale's 9th anniversary... but I'm exhausted (went to a local convention yesterday), and I don't know where to begin.
I guess one thing is... Chara and Asriel, who got a thinly veiled shoutout this anniversary.
I ran an Undertale theory blog on Tumblr back in the mid-2010s, and it was Chara and Asriel that fueled 90% of my fervor. Sans? Gaster? Only interesting to me when their lives intersected with the dead kids. The skeletons may be the easy-pickings theorybait, but the Tragedy of the Dreemurrs is the beating, bleeding heart of Undertale's story, themes, and lore. The grief carried in self-exiled Queen, the regrets of the King with the blood-soaked trident, and that grief seeps into the Underground even as nobody ever speaks the names of the King's first two children.
I think people tend to assume the metanarrative layer of a story that has one must be the most important layer - that it's inherently more real, more deep, more true than the silly simulacras of people acting out a story where we expect to take their world and problems as real. I've seen people reject, thus, the idea of even treating Chara as a character, because they think it's more deep if the "realest" layer of Chara is simply... a demon of videogame grinding, a manifestation of an external player's selfish impulses - not a character who had a past, who died and is grieved by a father-figure who reminisces about the hope in their eyes, who fell down a mountain in 201X, who liked chocolate and flowers just as much as knives, who was loved by their new family and hated the humanity they left behind, who made scary faces but also was camera-shy.
Personally, I think the best way to understand Undertale is that the metanarrative layer is in service of the more "in universe" story it tells. Undertale is a ghost story, of the hauntology of old sins and old wounds; the metanarrative exists to make the medium haunted. Asriel/Flowey and Chara are the unquiet dead, manifested as videogame creepypasta demons in the vein of "Ben Drowned." And I feel like that's fitting - they strike me as children the same age as the ones who read about Jeff the Killer and thought he was cool, who were fascinated by Pokemon Lost Silver and Buried Alive and Sonic.Exe, scaring themselves with amateur internet horror alongside all the anime and JPRGs that Asriel displays signifiers of in his Pacifist battle, or the literature than Chara quotes at one point in their narration of the cruelest run.
They dress themselves as what they find to be the scariest kind of malevolent spirit in-universe, just as much as Toby Fox wrote them in that mold in the real world to elicit a response. They conceptualize their detachment from the living in terms of the world being a replayable video game, with save states and finite dialogue, numbers and level-grinding; they just happen, on our layer of reality, to be right.
In some ways, Undertale's use of metannarative can be considered a successor to how 19th century horror utilized the epistolary format, and even outright "hoaxing", to make its ghoulish events feel more visceral, more believable, more real.
Undertale is not conventionally gothic, or purely gothic, but it is a story of the hauntological.
Hauntology, a category positing, presuming, implying a ‘time out of joint’,(21) a present stained with traces of the ghostly, the dead-but-unquiet, estranges reality […] with a radicalised uncanny – ‘something which is secretly familiar, which has undergone repression and then returned from it’ […]
Within the confines of the text itself, Asriel resigns himself to self-isolation by Chara's grave, feeling he deserves no better after all he's done. He fears that once the vestiges of souled-ness wear off of him and he returns to being a flower, he will be unsafe for other people; in his own mind, his story falls into the pattern of the undead bringing nothing but pain to their former loved onces.
(The Amalgamates went back to their families, and are loved and welcomed despite their uncanny new voices and forms; this is not a world where the undead have to be doomed.)
Just outside the text, he leaves the mountain. Flowey plays up his malicious nature, but he implictly is coexisting with others; Flowey is Asriel, and deep down wants many of the same things Asriel did, no matter how bitter and alienated he's become. And between him and Toriel, we see the fleeting ghost of Chara's memory - a child both of them knew, who was concerned with efficiency and rationality, and a little ridiculous for it. Asgore, even more briefly and discreetly, acknowledges them too.
Whatever form it takes, I wish the best of luck to Asriel and Chara in finding happiness and peace.