Howdy! I’m here to help YOU out with making a good Undertale OC! There are a lot of things you shouldn’t do in making a non cliche character, and I hope you appreciate this guide!

Species

Ok, lets start off with the species of your oc. If you want it to be a monster of some kind, make it any kind you want! A dog monster like Doggo, A  butterfly, A cat- thing, ANYTHING! Though the species I most often see in Undertale OCs are goats or skeletons, since the said species belong to the favored  character. I ain’t saying it's bad if you make those kind of characters! The sky's the limit when making ocs, though I recommend in not adding abnormal skin color or hair!

Appearance

Ah. The appearance. Appearances really depend on what kind of species they are, so for example; A skeleton usually  is made of bones (Duh) and wears  some  article of clothing (Except  for Gaster Blasters). And besides that, Skeleton’s can have  ANY sort of appearance! From a goat skeleton to a cat like skeleton, anything is possible! Now, onto the other species. Goats have white fur, horns, and usually grow up to be tall. Ghosts look like one of those ‘sheet’ ghosts and don’t take on a humanoid appearance usually. Robots usually vary in colors, sizes, and usually are humanoid in appearance. Humans usually are normal in appearance, and don’t have abnormally colored hair or skin. Also, mostly all children in Undertale  wear striped shirts of some color and kind.

(And a short note for Skeleton ocs; they usually have a glowy eye or something like that. I don’t know about the magic hair/genitals/tongue that much though. )

Special Abilities/Attacks

Now, the powers. Powers usually depend on what type of monster they are or what type of weapon they wield. Spear attacks for fish monsters like Undyne, Bone attacks for skeleton, or just leg attacks like Mettaton’s. Though, those attacks don’t have to apply to your oc (Though if your oc is in the royal guard those types have weapon like attacks.) Also,  skeletons/goats/whatever don’t have to have extremely powerful attacks or anything! Though if you want to make a oc similar to Sans in power, try to even everything out with it! Also, for skeletons; I don’t recommend using mini Gaster Blasters in their attacks. Your then basically stealing this from Sans himself.

Naming your oc

Oh-Ho-Ho! Naming. Names in Undertale are usually something related  to its name. Fonts for skeletons, Fire for Fire monsters, Dog related puns for dog-monsters, and some weird name for Goats(I’m not exactly sure for them.). And, don’t  forget  about nicknames they go by! If your oc only goes by some sort of nickname, how did they get it?  Did they make it up  himself? Someone else make it up? Try to do SOME research on fonts or dog related names, it wouldn’t be that hard to find any.

Family/Relationships

Now, this is something that slightly agitates me in all fandoms. ‘ So and So is the human brother of Frisk and is dating Sans’. Ok, I’m fine with canon x OC relationships. But please, ocs like that make me  cringe. If you want them to be ‘unique’, you don’t have to make them related to  another canon character. But if you want to do that, that's fine. Another thing; please, keep the canon character in character if your oc is dating said canon character! I don’t stress that enough since I see a character like Toriel acting totally ooc of her motherly character when dating someone (Just, anyone in general) and completely ignoring her children! Frhiuhegjtojigto.

Backstories

Now, onto the fun part. The backstory. People usually don’t go as ‘ in depth’ into their character’s past, or they get WAY too carried away. I just plead you guys NOT to make a cringey oc like; Asgore's brother was beaten and tortured for years by W.D Gaster until he fell into the void and then Asgore’s brother became the void. I’m not using ANYONE’s oc as an example for this, but this is something I often see. If you want to make a oc as a canon character’s sibling/lover/friend/whatever, then be realistic! Simple as that. Or if you just want to make a oc without ANY realtion to the canon story/characters, then take the same advice as I just told.