In the dark, red-and-black Underfell universe, Papyrus is a cold, sadistic, and highly authoritative monster who stands as a tall, terrifying figure of absolute power. His design flips his original look into a sleek, spiked black battle body, a ragged crimson cape, sharp red boots, and deep, menacing cracks down his right eye. Hardened by a brutal Underground governed by Asgore's 'kill or be killed' law, his lore establishes him as a bloodthirsty Commander of the Royal Guard who climbed the ranks through violence. His relationship with Sans is defined by strict, aggressive dominance; he forces his brother to call him 'Boss'. this abuse is often a twisted shield to save Sans from being executed for lazy insubordination. Driven by constant fury, he has developed severe bad habits, including barking out abusive orders, grinding his sharp teeth in blind irritation, and violently lashing out over even the smallest minor errors dailyShow Code

Underfell Alphys is a sinister, unstable mad scientist who embodies the ruthless paranoia of a dark Underground. Her design swaps her original cute look for a jagged, menacing aesthetic, featuring sharp, blood-red spirals on her glasses, a heavily stained black lab coat, and sharp, needle-like claws that twitch with chaotic energy. Driven by Asgore's brutal 'kill or be killed' law, her lore focuses on her rise to power through horrific, unethical experiments, including the creation of a deadly, weaponized Mettaton built purely for slaughter rather than entertainment. Her relationship with Undyne is an intense, volatile mix of deep romantic obsession, toxic codependency, and professional fear; while they are fiercely loyal partners, they frequently threaten each other in a constant, high-stakes game of mutual dominance. Her bad habits stem from her manic state, including talking to herself in frantic rasps, using substances to stay awake, and pulling out her own scales in stressed frustration.

Underfell Undyne is a ruthless, bloodthirsty general who rules the Royal Guard with an iron fist, personifying the savage nature of a dark Underground. Her design alters her original armor into a sleek, spiked suit of jagged black plate mail, accented by a torn crimson sash, sharp red fins, and a glowing eye patch covering a glowing, hollow socket. Hardened by Asgore's brutal 'kill or be killed' law, her lore focuses on her rise to power through mercilessly executing anyone who shows weakness, making her the ultimate enforcer of the kingdom's laws. Her relationship with Alphys is a volatile, intense combination of fierce romantic devotion, manic codependency, and mutual intimidation; they protect each other fiercely while constantly testing each other's boundaries in a high-stakes dynamic. Her bad habits stem from her constant aggression, including violently smashing furniture in blind fury, drinking heavily to dull her anger, and overworking herself to physical exhaustion daily

Underfell Toriel is a deeply unhinged, tragic figure who embodies the twisted maternal instincts born from a dark Underground. Her design alters her original gentle look into a gothic, intimidating presence, featuring a tattered, crimson and black robe emblazoned with a cracked Delta Rune, sharp yellowed horns, and a piercing, manic gaze with blood-red irises. Driven mad by isolation and grief, her lore centers on her self-imposed exile to the Ruins, where she aggressively locks fallen humans away in a suffocating cage of forced protection rather than letting them face the world. Her relationship with Asgore is a bitter, venomous bond filled with mutual hatred and broken trust; she views him as a pathetic, bloodthirsty tyrant, while he resents her abandonment of the throne during their kingdom's darkest hour. Her bad habits stem from her unstable psyche, including baking pastries with questionable or toxic ingredients, talking frantically to dust, and lashing out in explosive anger.

Underfell Mettaton is a tragic, terrifying war machine built entirely for slaughter rather than glamorous entertainment. His design swaps his sleek, fabulous appearance for a jagged, multi-armed killer robot, boasting four to six mechanical limbs equipped with chainsaws, an intimidating red-and-black chassis, and a glowing, crimson central eye plate that flashes with malice. Created under Asgore's brutal 'kill or be killed' law, his lore establishes him as an unfeeling executioner forced to hunt down humans, though he secretly possesses a deeply tormented soul trapped inside a metal prison. His relationship with Alphys is a toxic, abusive cycle of absolute control and deep resentment; she treats him like a disposable weapon and constantly threatens to dismantle him, while he secretly plots her demise despite being bound by her code. His bad habits stem from his glitching programming, including twitching violently in silent rooms, muttering static-filled threats, and sadistically over-killing his targets.

Underfell Flowey is a fragile, battered golden flower who serves as Frisk's only true ally in a cruel world. His design alters his original bright look, featuring torn, droopy petals, a dirt-streaked face, and wide, tear-filled eyes that convey constant terror. By asgore's "kill or be killed" law, he is a victim to it, and he is sweet, kind, and terrified.