Undertale OC Maker: Create Your Own Underground Character

Undertale carved out a permanent place in gaming history by doing something deceptively simple: it made you care about every character, from the final boss to the froggit in the first hallway. That emotional depth, combined with a world that feels simultaneously whimsical and tragic, has inspired one of the most creative OC communities in fandom. Years after release, people are still designing monsters, fallen humans, and alternate universe versions of the Underground with no signs of slowing down.

Oniichan's Undertale OC Maker lets you bring your Underground characters to life visually. Describe your monster, human, or something in between, and generate art that captures the charm, humor, and heart that define Toby Fox's world.

This guide covers everything you need to design a compelling Undertale OC, from SOUL colors and monster types to the unique aesthetic principles that make Undertale characters so memorable.

What Makes Undertale Characters Special

Before diving into the specifics, it is worth understanding why Undertale's character design works so well -- because those same principles should guide your OC.

Simplicity with personality. Undertale characters are not complex in terms of visual detail. Papyrus is a skeleton in a costume. Undyne is a fish person in armor. Mettaton is a rectangle with a wheel. But every design choice communicates personality.

Emotional range from simple elements. The game achieves devastating emotional beats with minimal visual information. A single sprite can make you laugh, cry, or feel genuinely threatened depending on the context.

Humor baked into design. Almost every Undertale character has something slightly absurd about their appearance or behavior. A skeleton who makes puns. A ghost who is too shy to move out of the way. A flower that is somehow the most terrifying thing in the game.

Choosing Your Character Type

The Underground is home to a vast range of beings. Your first decision is what kind of character you are creating.

Type	Description	Design Space	Canon Examples
Monster	Made of magic, form reflects soul	Virtually unlimited	Toriel, Undyne, Sans
Fallen Human	One of the humans who fell below	SOUL color + items	The six pre-Frisk humans
Ghost-Type	Incorporeal, can possess objects	Object + personality	Napstablook, Mettaton
Amalgamate	Fused monsters from Determination	Horror + sympathy	Endogeny, Lemon Bread
Darkner	Manifestations of real objects	Object-as-character	Lancer, Rouxls Kaard

Monsters

Monsters are the primary inhabitants of the Underground. They are made of magic rather than physical matter, which means their bodies are more expressive of their inner nature than any human body could be. A monster who is warm and nurturing might literally be made of fire. A monster who is cold and distant might be an ice elemental.

Monster designs in Undertale range from humanoid (Toriel, Asgore, Undyne) to abstract (Moldbygg, Memoryhead) to everyday objects given life. There are no rules about what a monster can look like.

💡 Tip: When designing a monster OC, think about what their physical form says about who they are. A monster made of crystal might be beautiful but fragile. A monster made of shadow might be misunderstood and feared despite being gentle.

Fallen Humans

Before the player character, six other humans fell into the Underground. We know almost nothing about most of them beyond their SOUL color and the items they carried. This deliberate ambiguity makes fallen human OCs incredibly popular because there is so much space to fill in.

Fallen human OCs should grapple with the central question of Undertale: how do you respond to a world that is afraid of you and may try to kill you? Do you fight? Do you show mercy? Do you try to find a third option?

Ghost-Type Characters

Napstablook and Mettaton (in his original form) represent a category of incorporeal monsters who can possess objects. Ghost-type OCs can inhabit anything from a suit of armor to a computer to a vending machine. The gap between their ethereal true nature and their physical vessel creates natural comedy and pathos.

Amalgamates

The Amalgamates are monsters who were injected with Determination and melted together into horrifying composite beings. They are some of the most disturbing and sympathetic characters in the game. An Amalgamate OC would be a fusion of multiple monsters, visually unsettling but emotionally vulnerable.

Darkners (Deltarune)

If you are open to drawing from Deltarune as well, Darkners are beings from the Dark World who are manifestations of real-world objects. Lancer is a playing card. Rouxls Kaard is a rules card. This opens up a design space where literally any object can become a character.

SOUL Colors and What They Mean

The SOUL system is central to Undertale's identity, and it should be central to your OC's design. Each color represents a core trait.

SOUL Color	Trait	Associated Items	Character Personality
Red	Determination	N/A (player)	Stubborn, resolute, refuses to give up
Orange	Bravery	Tough Glove, Manly Bandanna	Bold, physically oriented, possibly reckless
Yellow	Justice	Empty Gun, Cowboy Hat	Moral absolutist, sheriff-like
Green	Kindness	Burnt Pan, Stained Apron	Protective, gentle, self-sacrificing
Cyan	Patience	Toy Knife, Faded Ribbon	Quiet, observant, more perceptive than they appear
Blue	Integrity	Ballet Shoes, Old Tutu	Values authenticity and self-expression
Purple	Perseverance	Torn Notebook, Cloudy Glasses	Intellectual, methodical, deeply curious

💡 Tip: For monster OCs, SOUL color is less directly relevant since monster SOULs work differently. But you can still use these traits as a framework for personality design -- they are excellent shorthand for core character motivation.

Capturing Undertale's Visual Aesthetic

Undertale has a distinctive look that blends pixel art, anime influences, and a handmade quality that feels personal and intimate. Understanding this aesthetic will help you get results that feel authentic.

The Pixel-Meets-Anime Spectrum

Undertale operates on a visual spectrum. In-game sprites are simple pixel art with limited detail. But the game's battle sprites, official art, and the fandom's interpretations lean much more toward an anime-influenced style.

Your OC can sit anywhere on this spectrum:

• Pixel-art style portrait that looks like it was pulled from the game
• Detailed illustration that interprets what a character would look like in a fuller art style
• Somewhere in between that captures both aesthetics

Color Palettes by Region

Region	Color Palette	Mood
Ruins	Warm purples and reds	Nostalgic, safe
Snowdin	Cool whites and blues	Cozy, lighthearted
Waterfall	Deep blues and cyans, bioluminescent	Mysterious, melancholy
Hotland	Harsh oranges and reds	Intense, industrial
The CORE	Stark black/white with neon highlights	Sterile, dangerous
New Home	Grey and muted	Empty, somber

Expression Over Detail

The most beloved Undertale characters communicate through exaggerated expressions rather than intricate visual detail. Sans's permanent grin means one thing when he is joking and something entirely different during the Judgment Hall.

When designing your OC, prioritize a few strong, expressive features over complex outfits. A single distinctive visual trait -- like Undyne's eyepatch, Papyrus's scarf, or Muffet's five eyes -- does more work than a dozen small details.

Character Concepts to Get You Started

Here are some Undertale OC concepts that work well with the Undertale OC Maker.

1. The Skeleton Shopkeeper

A skeleton monster who runs a small curiosity shop in Waterfall, selling items they claim fell from the surface but which are mostly just interesting rocks they found.

• Look: Moth-eaten vest over a striped turtleneck, single gold tooth
• Personality: Tells terrible jokes but laughs uproariously at their own puns (unlike Sans)
• Secret: Their shop doubles as an informal counseling office because every monster comes to vent

2. The Goat Monster Historian

A young goat monster distantly related to the Dreemurr family who is obsessed with the history of the war between humans and monsters.

• Look: Round glasses slightly too big for their face, notebook stuffed with loose papers
• Haunts: Waterfall plaques, Gerson's shop, True Lab entries
• Quirk: Sweet and enthusiastic but brings up dark historical facts at inappropriate moments

3. The Fish Warrior in Training

A fish monster from the same species as Undyne who desperately wants to join the Royal Guard but keeps failing the entrance trials -- not because they lack strength, but because they cannot bring themselves to hurt anyone during combat tests.

• Look: Dented training armor held together with duct tape, slightly bent spear
• Arc: Finding a way to be brave and kind at the same time -- which is what Undertale is really about

4. The Reluctant Robot

A ghost-type monster who was supposed to possess a combat robot body, but the body arrived with a design flaw: it looks like a refrigerator.

• Conflict: Deeply embarrassed by their appearance while their cousin got the glamorous body
• Growth: Gradually learns that their practical features (storing items, keeping drinks cold) make them more beloved than any star could be

5. The Seventh Fallen Human

A human who fell into the Underground after the barrier was already broken but chose to descend anyway, curious about the monster world.

• SOUL: Green (Kindness)
• Weapon: A camera instead of a traditional weapon
• Journey: Documenting what remains of the now-abandoned Underground, meeting monsters who chose to stay behind

Tips for Great Undertale OC Prompts

When using the Undertale OC Maker, these tips will help you get results that feel authentically Undertale:

1. Lead with personality, not power level. Undertale is not about who is the strongest. Describe your OC's personality and quirks before their abilities. A shy ghost who apologizes for existing is more Undertale than a superpowered monster with seventeen attack types.

2. Include a touch of humor or absurdity. Find the funny angle in your OC's concept and lean into it. A spider who runs a bake sale. A skeleton who is an aspiring chef. The weirdness is the point.

3. Specify the area of the Underground. Mentioning Snowdin, Waterfall, Hotland, or the CORE gives the AI environmental context that affects lighting, color, and mood.

4. Reference specific Undertale characters for style. Something like "similar proportions to Undyne" or "same kind of simple round face as Sans" gives the generator a clear stylistic anchor.

5. Describe their encounter behavior. In Undertale, every monster has a unique encounter. Papyrus sets puzzles. Muffet traps you in webs. The dogs want to be pet. What does your OC do when they encounter a human?

Building Your Underground Story

The richest Undertale OCs exist within a narrative context. Think about where your character lives, who they know, and what they want.

The Underground is a small, interconnected world where everyone knows everyone. Your OC would have opinions about Mettaton's television show. They would have heard Papyrus yelling about puzzles. They would know about the human who came through and either freed everyone or left dust in their wake.

Alternate Universes

The Undertale fandom has a long tradition of AU creation:

• Underfell -- a darker, more hostile Underground
• Underswap -- characters swap roles
• Outertale -- set in space
• Your own AU -- with whatever rules and aesthetics you create

Build Relationships

Consider creating multiple characters that interact with each other. Undertale's strength is in relationships. Sans and Papyrus. Toriel and Asgore. Undyne and Alphys. A pair or group of OCs with a defined dynamic will always be more compelling than a solo character.

Start Creating

The Underground is waiting for a new arrival. Head to the Undertale OC Maker and design your character today. Whether you are building a skeleton with a bad pun habit, a fallen human searching for answers, or a monster who just wants to run a nice shop and be left alone, Oniichan's AI generator will help you capture the warmth, humor, and heart that make Undertale unforgettable.

Stay determined.