It's changed a lot over the years. It's mostly been carried on the backs of many, many different artists that come and go every few years. There's never really a central story or anything for them, mostly loose folklore. Here's a list and their wiki-pages for more reading.
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Heyuri-tan has been the poster child ever since she was made in i think 2021, and obviously recieved most of the OC cuz of that. - https://wiki.heyuri.net/index.php?title=Heyuri-tan
Iruyeh-tan is the evil troll sister with sad backstory. She hasn't come around in a little while. I miss her. - https://wiki.heyuri.net/index.php?title=Iruyeh-tan
Kaguya is... Kaguya. The admin. depicted as Kaguya Houraisan from touhou. (i like to call him kaguya-abi in my mind. the wiki page isn't that relevant)
/q/-tan is one you might see less often. I think she's cute. - https://wiki.heyuri.net/index.php?title=/q/-tan
faganon is a fat red guy. Usually associated with an annoying guy that i dont think posts here anymore, but is an umbrella annoying guy of Heyuri. - https://wiki.heyuri.net/index.php?title=Faganon
Puchippy is a character from Di Gi Charat. Her actual name is Puchiko, but back in September 2024 we had her as the mascot of a clippy like assistant that watched out for Rule 8 words when you made posts. It was annoying so we got her kicked out, but we keep calling her puchippy from that event. - https://wiki.heyuri.net/index.php?title=Puchippy

Waha is typically associated (here) with anonwaha, the admin of CGI@Heyuri and mod. Yamato Suzuran and Waha is its own, japanese internet meme. - https://knowyourmeme.com/memes/yamato-suzuran-waha

You'll generally see alot of japanese internet memes in OC.

Some of the characters you might see in older OC:
Hatsune Miku - This is lolico, the founding admin. Don't see her as much anymore since he left about 3 years ago - https://wiki.heyuri.net/index.php?title=Lolico
Tokino Tomo - This is kuz, the other admin. More recent and was around for stuff like the Golden Age of Heyuri and also a bit of a drama magnet. Led to quite a bit of the OC and stuff you see today though. https://wiki.heyuri.net/index.php?title=Kuz
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The wiki page for OC is a little outdated, but it's still nice if you wanna have a look for a bit of the old stuff. - https://wiki.heyuri.net/index.php?title=Original_content

>>155406
I fucked up the scroll tags. (ﾟ血ﾟ#) I'm killing myself TONIGHT ヽ(`Д´)ノ

>>155406
Thank you!

That was a fun read.

>>155406
woah half the iruyeh page is straight from my thread (;ﾟДﾟ)
i didnt know my drawing had such an influence

This thread is relevant to my interests

Japanese Tourist Guy

He taek picturez

"Iyaaa～h! The wind is lifting up my skirt!"

"YOSHI, IKUZO"

>>155406
you say you miss iruyeh, if so why did you not seek her out in her moment of obscurity?

>>155451
WAAAAHHH! IT'S IRUYEH TAN!!! Σ(ﾟдﾟ|||)Σ(ﾟдﾟ|||) EVERYONE RUN!!!

>>155451
I... uhh.... Σ(;ﾟДﾟ) i was... busy! uhm.... Heyuri-tan needed some help, and i had to get busy!

>>155456
>>155457
mattaku... don't you guys realize it's exactly this kind of behaviour that is making iruyeh-tan more hostile towards us?! ヽ(`Д´)ノ

>>155451
Iruyeh-tan, how nice of you to come. Please, have a seat wherever you'd like. I'm brewing some tea, would you like some? (´ー`)

>>155461
please excuse her, she had some bad experience with previous heyurizen (;´Д`)

ｷﾀ━━━(ﾟ∀ﾟ)━━━!!

>>155483
Ok, it's boiling in the kettle. While we're waiting for it to heat up, would you like to talk? I want to hear how you've been. It's been so long since you visited your old man, you know. ( ´ω`)

>>155406
Thank you!

>>155389
Putin

>>155586
he forgot about dovegirl, if only heyuri cared more we'd have her entire lore done by now

>>155621
dovegirl is before my time... or so i thought, i just never saw her nor have her immortalized in the wiki... I'll get her archived tomorrow. DOVEGIRL SHALL NOT BE FORGOTTEN!!!! ヽ(´∇`)ノ

>>155496
i enjoy these iruyeh-tan roleplays we have when i post my drawings (´ー`)

>>155629
me too!

Oh, it's still giving me trouble, hehe~
But such things can't be helped when you get to my age. Ah, I hate to trouble you, but would you be a dear and answer the door? I'm expecting another visitor soon.

*knock knock*

A silhouette can be seen be seen through the door window. From the front, it almost looks like... a star?

>>155634
a new friend has arrived! let's all greet her warmly!

>>155646
Heh. Goodness, she's really too much for this old coot to keep up with...

*door opens*

Welcome home, little one. I bet you were working all day preparing for that Bunkasai thing you young'uns are so excited for. Be sure to take a break and rest up now. Oh, and say hello to Iruyeh-tan. I think you two have met before... now when was that... hrm...

Ojisan is lost in thought. What happens next?