Theory: Sunny is at least part Chinese
Okay so the general fan consensus is that he's Japanese, but I think I've found (shaky) evidence that he's Chinese, at least on his mother's side.
Othermart Lady's dialogue. Sunny's able to decipher parts of her Mandarin. Some may argue that she's speaking partial English, but then why speak Mandarin at all? The way her son Thick Eyebrows speaks gives me a sense that he's been in America since he was a kid, which means realistically Othermart Lady should have picked up English at at least a limited level. You might say she has, but a lot of her dialogue is Mandarin for basic phrases like "Do you want". Alternatively, the fact Sunny recognises it as Mandarin specifically is odd. Faraway is a low diversity town, I don't think Sunny would have any exposure to differentiate Japanese, Mandarin or Korean. If Omocat just wanted it to be another language, she could've used a mix of non-latin scripts, or just say "They're speaking in another language." Either way, Sunny is at least familiar with some Mandarin, and I kind of find it odd that he would since he didn't go to high school and wouldn't be able to pick up Mandarin as an elective. Unless he went through Mandarin tutoring when he was young, which would make sense if he was part Chinese.
Othermart Lady's name. It isn't one, or rather it's more of a title. Her name in the game files is Ayee, which is a transliteration of A yi which literally means Auntie, but can be used on any older lady in Chinese. There's not really anywhere Sunny could've picked this up from. Joy, Michael and Thick Eyebrows wouldn't call her that since she's their grandmother and mother respectively. And earlier, Sunny could barely decipher Mandarin so I doubt he picked it up from someone else anyways. It's very likely Sunny picked this up from someone telling him to "help buy fish from Ayee" or something along those lines.
His mother's hair colour. Red hair is extremely rare, particularly among Asians. Plus, Sunny's parents had no qualms about Mari dying her hair, so I find it likely Sunny's mom dyed hers red. Red is an auspicious colour for Chinese people and it's popular for Chinese women to dye their hair that colour. And in the one place we see his mother, she has red dyed hair and is wearing red.
Omocat is Chinese-American. Yeah that's all I got lol.
COUNTER ARGUMENTS:
The mitamaya in his house. Firstly, it's barely even a shrine tbh, it's a photo and 2 candles. Secondly, it's neither a point against or towards him being Chinese since both cultures have burning candles for the dead.
Mari 's name is Japanese. It isn't really. It has a ton of possible linguistic origins. Either way, only her mother's Chinese in this theory. Her father could be Japanese and name her Mari
OMORI's name... Is from the piano. It's the name equivalent of Yamaha.
OMORI's favourite food is onigiri. Let the man eat what he wants.
That's all I got for now. Let me know anything I missed.