Like many works that trace Frisk’s journey through the Underground, the plot is pretty simple and linear. Frisk rewinding time when killed slightly complicates the timeline, but Frisk has only died three times so far.
As Underfell works go, the work is mild. As expected, there's sometimes blood (though rarely in large amounts), and plenty of swearing. Despite the setting, characters are often mean, cynical, and aggressive rather than outright evil or murderous, and are easily pacified. Situations are also rarely very frightening or violent. Along with its simple, rounded, relatively well-lit style, it's much milder than HorrorTale or the famous Flowey is Not a Good Life Coach.
Characterization
Frisk
Given the situation, Frisk is rather bold, though still friendly and pacifistic. At first, they think they’re dreaming, explaining their unworried approach. However, when Toriel kills them in an escape attempt, they realize they’re not dreaming and break down, sobbing. Yet they quickly recover after dreadful realizations, fights and even death, becoming a happy camper once more. Literally: it's like Frisk is just an experienced camper lost in a bear-filled forest, rather than a strange land of monsters trying to kill them. They even feel safe enough for Flowey to read books and history plaques to them, despite Flowey’s reminder they’re in a hurry. (This might be explained by Frisk being too young to understand the danger they’re in, or realize their power makes them unkillable)
While friendly and forgiving, sometimes Frisk is insistent, such as yanking Flowey out of the ground to put him in a boot and thus carry him into Toriel’s house.
Flowey
In many Underfell works, monsters' morality is basically flipped: good characters are now 'evil'. Thus, the murderous, sociopathic Flowey is now a helpful companion, often carried by Frisk. Though that's true here, Flowey is not sweet or innocent. He is prone to anger, frustration, and indignant outbursts, and sometimes intimidates weaker monsters with a scary face. He says he "can't console people", and smacks Frisk to make them stop sobbing. Flowey knows "in this world, it's kill or be killed", but is sad and resigned about it. Overall, he might be as Flowey once was before choosing murder runs.
In this work, Asriel was once a jerk: he took Chara to his parents while eagerly asking about killing them. Thus, it's likely Flowey is only 'good' because Asriel was once bad. Notably, Flowey mentally calls himself 'Asriel', apparently only coming up with 'Flowey' when Frisk asked for his name.
Toriel
As expected, Toriel is meaner and more violent than in canon. Indeed, Flowey is afraid she'll kill him and Frisk if she spots the two of them, and is surprised when she doesn't. It's likely she outright kills monsters in the Ruins: Flowey notices one hallway is "dustier than usual", and afterward her clothes have dust stains. She’s more disdainful of Flowey than in canon, calling him a 'weed', threatening him, and initially refusing to let him into her house.
Her sadness and long isolation apparently damaged her social skills, or even sanity. Her pie is very burnt, and has so much cinnamon it makes Frisk cough. Having run out of butterscotch, she put in monster dust instead, which isn't at all reasonable. Though she would surely know how bad the pie was, she aggressively insists Frisk eat it and not even think of throwing up afterward. When Frisk tricks her to escape, she calls them ungrateful and burns them alive at the exit.
Yet, she can still be kind: she sheltered Frisk, gave them food, tucked them in and gave them a plushie as they slept. (Though she glares at Flowey for noticing the latter two) Frisk points out her kindness in the fight: they say they don't really want to leave, and that's she's not evil, just lonely and sad. Toriel is stunned. She breaks down, sobs, and says: "Why are you being...so nice to me?” However, she is so deranged she soon "realizes" Frisk's kindness is a trick, a prelude to greater horrors, and she chases them out of the Ruins.
Papyrus & Sans
Sans hasn’t actually changed much. He has a taste for meaner, darker jokes. Compared to other characters, he is uncommitted: he’s neither especially helpful nor harmful to Frisk, though a few of his actions could have gotten Frisk hurt or killed anyway. While he does kill Frisk with an electric shock from a joy buzzer, when Frisk reloads and refuses to shake his hand, he guesses that the buzzer would have probably killed them anyway.
Here, Sans definitely knows about Flowey. Though he knows Alphys had been looking for Flowey, he didn't report him: he found it more entertaining to keep seeing Alphys throw a fit. (Notably, he asks Frisk's name, unlike in canon.)
Papyrus is even more confident and egotistic than before, as shown by the even more muscular snow sculpture of himself. While canon Papyrus never dismissed others for his self-enhancement, this Papyrus does just that, calling Frisk a "pathetic weakling". He is still a skeleton with high standards: while more vicious, he insists on fair puzzles and fights. (despite talking about how much he'll benefit from capturing Frisk)
Art
Left: The first page. Right: A re-do of the first page, showing improvements in shading and lighting.
Though not as polished and precise as other comics, the art improves over time. Indeed, there's a big jump in quality on the last page of the Snowdin arc and later in Waterfall, too. The lineart is crisper, the shading better, and the colors richer. (if very purple) While the hand-lettering has tight and irregular spacing at first, it improves a lot midway through the Snowdin arc.
As appropriate for an "edgy" setting, characters are toothier and spikier, to the point even Moldsmal and Papyrus's house (using icicles) are spiky. As expected for Underfell, characters look scarier, with duller, darker colors. Red and black (unsurprisingly) dominate the work: not only are many characters’ clothes red and black, but even attacks are often red. (Except fire, oddly enough: that's blue instead.) Colors are changed to darker, duller, and redder shades overall.
The right image illustrates about half the colors in Snowdin.
Often, this results in shades one could broadly call "purple", especially reddish purple.
For example, the walls of the Ruins are reddish-purple, and sometimes Snowdin's snow is purplish-taupe. The purple-ification is especially obvious with the Echo Flowers (once cyan, now phlox), Waterfall (once dark blue/indigo, now dark bluish purple) and the gratuitous purple Moldsmals. Indeed, it's so purple one could easily call this work "that purple Underfell".
Yet, one could also call this "that low-green Underfell". Other than Flowey's (often dull) green stem, there is practically no green in the Ruins. Things get greener in Snowdin, though the trees are a fairly dull shade. (Oddly so: the trees were actually blue in-game). In Waterfall, green becomes rare once again.
Yellow and orange are rare, too, if less so. Frisk's skin is not mango-yellow, but an only slightly yellowish beige. Monster Kid is mostly a washed-out orange (who gets even less yellow by Waterfall), and wears red-and-black clothes. Even monsters' outfits have less yellow, such as Papyrus, Shopkeeper Temmie and especially the Nice Cream Man. In some cases, though, the work adds yellow: Napstablook and Toriel have yellow eyes, Sans has a yellow tooth, and Undyne's armor has yellow accents.
One can get used to the limited color scheme over its hundreds of pages. But the bright, green-and-yellow palette of Flowey’s dream sequence in Page 126 and Waterfall’s yellow history-plaque flashbacks remind the reader of the limited colors.Even the comic itself lampshades its dark, limited palette: Papyrus's color tile puzzle is various shades of grey, black, and white.
Conclusion
For those who want something a little scarier and unhappier than canon Undertale, but aren't quite ready for darkfic or standard Underfell approaches, Kaitogirl's Underfell is a perfect middle ground.
IT SUCKS IT'S SHIT! I SUCK AT THIS KIND OF THING.... I'M SORRY!!!!!!!!
I WILL MAKE A REMAKE ONCE I GET A WHOLE LOT BETTER!
Who is this human? what is she doing here in the underground of hell?! find out next time in PANDORAFELL!
