Fell

Papyrus (younger brother). Papyrus acts like a "boss": he commands, belittles, demands that Fell call him "boss," and generally puts pressure on his brother. In Underfell, Papyrus isn't sweet and naive, but domineering and harsh.
Fell is patient and protective: he constantly smooths over rough patches, cleans up the mess left by Papyrus's antics, and defends him, even when he himself is tired and angry. This isn't out of admiration, but out of a sense of responsibility: "He's my brother, so I'm responsible for him." Mutual care is hidden behind rudeness: if Papyrus is in real danger, Fell doesn't hesitate to rush to his aid. And Papyrus, though he presses him, still doesn't want to actually harm his brother.
Everyday details: Sometimes Papyrus makes Fell help with "training" or patrols, and Fell grumbles but does it.

a harsh, tired and embittered character.

Main features
Bitterness and cynicism. He's furious at humans for locking the monsters underground, and he doesn't hide his contempt. His remarks often sound harsh rather than lightly ironic.
Fatigue and irritability. He's constantly exhausted, so he reacts harshly, and can snap even without much provocation. He's not so much "angry to the core" as he is a person (a skeleton) who's fed up with everything and is holding on with the last of his strength. Restraint and composure. Despite his quick temper, he tries to maintain control and act prudently. His emotions sometimes flare up, but he suppresses them because "now is not the time."
A rough-and-ready care for loved ones. This is especially evident in his relationship with Papyrus: his brother constantly belittles and bosses him around, but Fell tolerates it and still covers for him, albeit grumblingly. There's affection, resentment, and weariness in this, as well as the role of "the one who sorts things out." Sarcasm, but without the humor. For the original Sans, puns are a way to lighten the mood. Fell's jokes are either mean, dry, or not perceived as jokes at all. He's not amused.
A sense of duty and responsibility. He takes on difficult work and protects those he considers "his own," even if he does so reluctantly and with a disgruntled expression.
Loves mustard.

A skeleton with sharp teeth, one of which (the upper left fang) is golden.
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A red eye that constantly glows, symbolizing his emotional state
Heavy black coat with a gold zipper.
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Red sweater with a gold chain.
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Black trousers with vertical yellow stripes.
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Red and white sneakers with yellow socks.
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Ruffled fur on the hood.
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Sometimes a crack is visible on the skull (probably due to conflicts with his brother)
Black fingerless gloves