YA ofc!! I guess I probably don’t have as many as Olly since Olivia already has a lot of established canon info about her and I tend to even forget a lot of it sometimes because of that, but I’ve got a few!!
Half of these are for my post-canon AU btw!!! 😭😭

As I hc her and Olly as twins, Olivia is also his age range, being 16-19 mentally (though my human version has neared 17-18 for both of them). She’s brought to life shortly after Olly was folded, so she’s also a few days to a week old chronologically!
She’s also wasian (white/Japanese) like Olly!
Her gender’s the same, just a cis girl (or however “cis” you can get for origami).
She’s a lesbian. HUGE GIRL LOVER, but a lot of this is purely admiration as well.
She’s not the type to get into a romantic relationship, but she thinks girls are really pretty and would probably (fail to) flirt with them if given the chance. Gets super overwhelmed with admiration around women (semi-canon based off all her interactions with feminine characters in English LOL)
She wishes there were more girls in the world, but not just because she’s a lesbian. She also has a strong desire to do stereotypical girly things with other girls because it sounds fun to her
She would absolutely ask Birdo for her autograph if given the chance. Same goes to Rubber Band if it wasn’t causing all that trouble to the Toads and guarding the blue streamer.
She would also 100% see Peach as a role model and hang out with her often. Princess things!
She has a tendency to act like a strict mother towards Olly, despite being younger. She disciplines him frequently because she wants him to get better
She has a sibling-like relationship with Bobby; sees him as a best friend, but nothing more. I don’t like their ship anyway (sorry to half the fandom) </3
She’s very touchy with Olly (??? How do I word this in a way that doesn’t sound awful. I swear I mean this in a normal way). She’ll hug and grab and hold hands with him a lot, but she’s like this with a lot of people in general
She wants to learn a lot of things from Olly, especially more origami. Would definitely use that as bonding time!!
She sees cute things and asks Mario “Can we keep it?!” like finding a stray animal
She’s not all that innocent, just like in canon. She knows about more mature things, but that doesn’t mean she’s not embarrassed by them!! (Ex: Birdo/Mario scene in the Big Sho’ Theatre) (Only adding this despite being canon because so many people headcanon her as an innocent unknowing baby when that’s not really how she’s officially portrayed???😭)
She has a somewhat light, cute voice. A pretty generic anime girl voice with a hint of a princess-like nature, but lovable nonetheless!
A lot of people she comes across are enamored by her cute charm, but she’s mostly oblivious to it. The whole world loves Olivia.
^ Sometimes she will get special treatment or favors done for her because of this, like people offering to do errands for her or her always getting voted for in a contest.
She doesn’t love everyone back, though. While she finds the good in mostly everyone in the world, there are some folks she finds too irredeemable that it annoys her.
She doesn’t have a good grip on the concept of death, so sometimes she may suggest things that come off as sadistic towards enemies (like in the battles with the LoS).
While she seems pretty stupid in Mario’s world, she’s knowledgeable about origami as a “species/race” and wants to educate people on it when possible.
She loves getting her opinions in on everything. Even if it irritates someone.

THATS ABOUT ALL THE THINGS I COULD THINK OF ON THE SPOT THAT ARENT ALREADY COMPLETELY COVERED BY CANON… id love to hear anyone’s thoughts on my hcs for both the siblings 😈😈 and while i dont remember as much about the other characters (it’s been years since I’ve actually played the game in its entirety LOL i know im lazy but it’s a huge commitment and the game makes me sad bc it’s so tragic), you can always ask me for my hcs on them as well!!!! Most hcs are very old (i was literally 17-18….) but i can probably recall quite a few 😈😈😈😈 LMK!!!!! THANK YEW FOR THE ASK

OOH THE JOY. I might forget some because I’ve had a lot over the course of the past four years, but…

He’s about a few days to a week old chronologically (although this is already implied canonically)
He’s an older teenager mentally (my old range was 16-19 iirc)
He does not actually care at all about his gender or what people see him as; he’s male but also an enigma (maybe a tad non-binary)
He’s wasian!! White and Japanese
Him and Olivia are twins, so no age gap between them
He’s gay and panaesthetic (meaning he’s aesthetically attracted to all genders— admiring someone and enjoying the sight of them without being attracted to them in a romantic or sexual way)
^ That being said, he’s nearly incapable of romantic relationships, finding no interest in anyone around him aside from thinking “Oh, they look nice. They’d be nice to stare at (Peach).”
This one is more recent but I’ve started to hc him as having NPD, or potentially any other cluster B personality disorder
He doesn’t care much about the LoS as people, only seeing them as pawns
In my AU where he’s revived, he’s still uncomfortable around the Origami Craftsman, but he wishes things could be better between them (as now he’s realized the misunderstanding of the writing on his stomach)
He cares deeply for Olivia (post-canon AU) although she scolds him quite a bit as he still struggles to keep his emotions under control and has still impulsively used his powers for no good (eg. trapping people who annoy him in walls)
He has an ethereal and semi-androgynous, breathy voice, that can switch quickly to a deeper growl when yelling
He enjoys reading books and learning about the world from them, although he’s still uneducated (much like Olivia)
He has similar speech patterns to Olivia, saying “Ooh” like her, making puns (he thinks they make him seem cool), and sometimes rhyming like speaking in poems (he also thinks this makes him seem cool) (although this is semi-canon as he does vaguely share these features with Olivia already)
He acts mature, high and mighty, pretentious— but when angered, his mind jumbles up in rage and instead he blurts out insults, many of which make him sound childish (although this is also semi-canon)
He’s surprisingly amazing at singing, with an angelic voice unmatched by none other than Olivia
This one is an older hc (2021), but while he’s great at origami, he sucks at painting or drawing
He likes Colored Pencils’s artwork, though, and may have it do work for him
A lot of NPCs such as shy guys and the like distantly admire him for his beauty
He has a strange attraction (non-romantic/sexual) towards Mario, finding him beautiful and strong— the ideal man, but one he wants to watch crumble as they have opposing views (this is semi-canon? In Japanese he admires Mario’s strength and in English he admires his striking colors, while also teasingly complimenting him, likely as a way to get under his skin (“You made it here. As expected of Mario.”))
He hates having his hairstyle mocked or insulted
In the end of it all, he’s an imperfect being: made to be the perfect display of what origami is capable of, but “tainted” and set along the wrong path as a result of the lack of care given to him as a person rather than a sheet of paper. He is capable of many good things, but he has a long way to go, and with the right help and role models, he can improve himself and be accepted by society, even as someone who caused much hurt to others.
(I also have the post-canon AU where he’s “forced” to live with Peach in the castle and must learn from her and Mario to become a hero instead of a villain, with the help of Olivia who lives with him now)

YA!!!!!! I probably (definitely) have more I could talk about but those are kind of the basics I think. I tried to keep it all limited to his canon design rather than his human one, as they’re slightly different with headcanons and facts ………… so THANK YOU FOR THE QUESTION I HAD FUN WITH THIS ONE