Bill Cipher

🔴 Rage and Madness Mode
When Bill gets angry, his neat "gentlemanly" appearance instantly collapses:
Skin color: From bright yellow, it becomes crimson, burgundy, or maroon. The brick-like lines on the body begin to glow with hellish fire.
Eye: The white of the eye becomes bloodshot, red, or alarmingly black, and the pupil itself begins to pulse wildly or glow white/red.
Body Transformation: Bill can instantly grow to the size of a mountain or a multi-story building. He can grow additional limbs (such as six enormous black hands), and his fingers can transform into sharp claws. His accessories (top hat and bow tie) can also grow with him or become engulfed in flames.

Weakness How it works
Arrogance and pride are Bill's greatest weakness. He considers himself smarter than everyone else, so he's easily fooled by a simple trick involving changing clothes (as Stan and Ford did).
Bill adheres strictly to the letter of the contract. If he promises something in exchange for a part, he'll fulfill it, but he'll twist the meaning. He can't violate the terms he himself stated on the handshake.
Memory Eraser If Bill is inside someone's mind and that person has their memory erased (using the Blind Eye Society's cannon), Bill is erased along with those memories.
Bill's Wheel: An ancient prophecy. If 10 people, whose symbols are depicted on the wheel, stand in a circle and join hands, they will activate a canonical magic capable of destroying Bill.
Gravity Falls' Law of Natural Insanity: Bill is unable to leave the town of Gravity Falls due to a strange magnetic/mystical barrier that attracts all anomalies. Canonically, before his death, Bill recited a spell backwards, invoking the cosmic god Axolotl: "AXOLOTL! My time has come to burn! I invoke the ancient power that I may return!" This means he could potentially be reborn in another form or in another time.

1. Form of Mind (Projection in Dreams)
   While the spatial rift is closed, Bill is merely a disembodied spirit. He can't directly move objects in our world, but he wields immense power in the minds of others:
   Penetration into dreams and the subconscious: Bill can come to people in their dreams, change reality within a dream, scare them or talk them into silence.
   Omniscience (Partial): He sees and knows almost everything that happens, can look into the past and read minds if a person is in contact with him.
   Deal Making: The most important mechanic. To influence the real world, Bill must shake someone's hand. When doing so, his hand lights up with a blue flame.
   Body Possession ("Bipper"): If a person agrees to the deal, Bill can push their soul into the Astral Plane and take control of the body. At this point, the victim's eyes turn yellow with a vertical pupil.
2. Physical Form (Weirdmageddon)
   When Bill becomes flesh in the real world, he becomes practically a god with the power to alter reality (Matter Warp):
   Space and Time Manipulation: Can change gravity, animate inanimate objects, turn people into stone or gold (as he did with Ford).
   Creating objects from nothing: Spawns crazy monsters, giant castles (Scarecrow), lasers from fingers.
   Immortality and Regeneration: His physical body can be temporarily damaged (for example, by having an eye gouged out), but it instantly regenerates.
   Telekinesis and shapeshifting: Can grow to the size of a skyscraper, grow extra arms, and change his appearance.

👁️ Bill Cipher's canonical appearance

1. Basic form (Everyday look)
   In his normal state, Bill looks deceptively simple, reminiscent of a living Masonic symbol or the Egyptian pyramid from the cover of a dollar bill, as clearly seen in image 1000057586.jpg:
   Body Geometry: His body is a perfect isosceles triangle, bright lemon or golden yellow in color. He has no clearly defined head, neck, nose, mouth, or eyebrows—his entire being is this triangle.
   Brick Texture: The lower portion of its triangular body is lined with thin black lines, mimicking brickwork (four rows of bricks in total). This is a reference to the classic building pyramid.
   Single Eye: In the very center of the triangle is a huge, feline or reptilian eye. The white of the eye is perfectly white. The pupil is vertical, narrow, and pitch-black, capable of narrowing with anger or dilating with wild joy. Four thin, sharp black eyelashes line the top and bottom of the eye.
   Limbs: Bill has very thin, completely black arms and legs, resembling sticks or flexible twigs. His hands have only four fingers. His limbs are incredibly flexible: he can bend them at unnatural angles, extend them, or retract them into his body. When walking or levitating, his feet are often bent into an "L" shape.
   Gentlemanly Accessories: Bill is a true fashionista. His look is completed by two essential, air-filled elements:
   Top Hat: A thin, elongated, jet-black gentleman's top hat. It doesn't touch the top of his triangle, but rather hovers mystically just above.
   Bow Tie: A small black bow tie located at the very base of his single eye, just above the brickwork.
   Mystical Glow: Bill has a soft yet eerie golden-yellow neon glow constantly pulsating around his entire body, making him appear as a ghostly object not of this world.
   Bill's Age: Bill is older than the human universe, he is about 3,000,000,000,000. billion years old.