Ye Yu

Biodata

Power Progression

Background / History

Ye Yu was an ordinary person on Blue Star before a runaway dump truck struck and killed him. A hostile consciousness selected him as its vessel, placed his consciousness in a black-and-white body, and deliberately omitted a soul. Its larger plan was to erase and restart the universe after failing to expel Linde 33.

Calling itself the Self-Error-Correction System, the entity convinced Ye Yu that monster technology would threaten humanity once monsters escaped the barrier. It identified Linde and Frisk as his principal obstacles and encouraged him to gather LOVE. When Ye Yu questioned his missing emotions, the System claimed it had sealed them to preserve his reason. In reality, it wanted to eliminate his pity, and LOVE could still corrupt his consciousness despite his lack of a soul 36.

The System sent a Subsystem into the Undertale world alongside him. Although it provided combat assistance, its primary purpose was to monitor Ye Yu and prevent him from changing sides 40 50.

Appearance

• Black-and-white body: Artificially fashioned by the entity; Scarlet describes him as resembling a human drained of color 33 42.
• Crimson eyes: Initially described as emotionless. Later appearances emphasize red flashes and, eventually, entirely blood-red eyes filled with bloodlust 33 42 47.
• Red blood: Sans notices that Ye Yu bleeds red despite his differences from other humans 49.
• Expressionless face: Remains largely unchanged even while killing opponents or assessing danger 40 41 42.

Personality

Initially, Ye Yu retains the ability to reason and question his circumstances but feels neither guilt nor ordinary emotional attachment. He suspects the System might be lying about his emotions, yet accepts its explanation. He approaches early battles analytically, studying opponents and considering solutions in the Void Space between attempts 36 40 42 43.

LOVE progressively replaces this reasoning with compulsive bloodlust. Thinking about slaughter relieves his lingering discomfort, and reaching LV 10 intensifies his desire to kill. After defeating Gaster’s followers, he forgets an earlier intention and recognizes slaughter as his only purpose 41 42 45.

His corruption does not erase his tactical intelligence. He identifies North Star as the most dangerous member of a three-monster team, exploits openings with feints, and remains wary when Sans disappears during the skeleton battle 47 50. Brief reactions also persist: he freezes when Martlet recognizes him as human, and Mettaton relays Alphys’s belief that being soulless does not necessarily mean losing every emotion 46 49.

Abilities & Skills

Determination and Reloading

Determination allows Ye Yu to return after death and retain knowledge from previous attempts 36 40 49.

• Returns to the Void Space after dying, where he can analyze a fight before loading or pressing Continue 42 43 44.
• Learns attack patterns across repeated battles, including his encounters with Decibat and Sans 40 49.
• Undyne can sense timeline changes, while Sans retains only a vague feeling of familiarity after a reload 43 49.
• Reloading does not guarantee an easy victory: Undyne varies her attacks between attempts and defeats him more than forty times 44 45.
• Determination heals his injuries after Mettaton’s self-destruction leaves him barely alive 49.

Soulless Body

His lack of a soul protects him from attacks directed specifically at the soul, but also prevents him from using ordinary magic 40 42.

• Decibat’s soul-targeting sound waves have no effect 40.
• Physical attacks can injure or kill him, a weakness Darbat broadcasts to the monsters 41.
• LOVE increases his physical attack and magical defense much more than his physical defense; Sans’s physical bones remain lethal despite his higher level 49.
• Determination-infused and data-based attacks can bypass the protection that frustrates conventional monster attacks 44 45.

Direct Determination Manipulation

Ye Yu learns to apply determination beyond reloading, although controlling it requires exceptionally strong willpower 42.

• Restrains and crushes Scarlet’s intangible ghost form 42.
• Charges his blade to produce a broad arc capable of destroying incoming spears 43.
• Solidifies determination beneath his feet to change direction in midair 45 50.

Combat Adaptation

Repeated deaths develop his speed, evasion, and tactical precision 44 49 50.

• Cuts Scarlet more than twenty times in one second 42.
• At LV 15, converts accumulated combat experience into increasingly efficient killing techniques 47.
• Uses punches, kicks, blade feints, and black energy waves against Mettaton and her drones 49.
• Demonstrates teleportation while setting a trap for Papyrus 50.
• Learns to evade Orange Bones after only two deaths 50.

Guaranteed-Hit Slash

A delayed attack that draws on the world’s rules and lands after the visible swing has ended 50.

• Kills Papyrus while Sans and Semi are possessing him 50.
• A single use leaves Ye Yu utterly exhausted, so he rarely employs it 50.
• Its delay limits its usefulness against suspicious opponents; Ye Yu judges that Sans would not remain in place long enough 50.

Reduced Hitbox

The Subsystem rewrites the world’s rules specifically for Ye Yu, reducing his vulnerable area to a tiny point on his abdomen 50.

• Makes dense barrages substantially easier to dodge.
• Each successful hit removes one-fifth of his HP rather than instantly killing him.
• Requires adjustment; he dies several times before learning to use the altered hitbox effectively 50.

Equipment / Weapons / Items

• Erasure Knife — A long blade initially shown flickering with code errors. The Subsystem expends power to stabilize and strengthen it against Gaster’s followers, allowing an invisible slash to strike all four 40 45.
• Armor — His first displayed status panel lists no armor 40.

Relationships

• Self-Error-Correction System / Error-Checking System — Creator of his replacement body and manipulator of his mission. It falsely presents emotional suppression as protection while intending to turn him into a killing machine 33 36.
• Subsystem — Accompanies, advises, and monitors him. It sacrifices its power first to strengthen his knife and later to alter his hitbox 40 45 50.
• Linde — Principal assassination target. Ye Yu blames him for the monsters’ unexpectedly altered abilities and vows to kill him 36 44.
• Frisk — Identified as another major obstacle. Asgore confines Frisk and Linde because Ye Yu could absorb their souls and rewind time 36 40.
• Undyne — Repeatedly defeats him and forces him to refine his evasion. Monster Kid intercepts Ye Yu’s finishing blow and enables her escape 43 44 45.

Story Role / Major Arcs

Notable Quotes

“...I suppose I don't really have a choice either...” 41

“Something's wrong with this world. These monsters' abilities are far too strange. Even though I never played Undertale in my previous life, I'm certain their abilities weren't supposed to be like this.” 44

Trivia

• The author explicitly describes Ye Yu as an originally innocent, “pitiful pawn,” and states in the Chapter 33 note that whether to kill or save him had not yet been decided 33.
• Chapter 40 shifts the story’s perspective to Ye Yu, presenting Linde and Frisk as his “final bosses” 40.