A/N: I'm using a female MC for this, since it's easier for me to write a female character.

Riddle Rosehearts
At first Riddle just thought she was a shy person. He had a feeling that it wasn't the case, but couldn't be bothered to ask her. Prior to his overblot, he hardly spoke to her since she was in Ramshackle dorm. However, he noticed that she regularly avoided eye contact with everybody around her. He just found MC awfully passive.

The first time he saw her at one of the Unbirthday Parties, he noticed she often spun around or paced back and forth, occasionally fidgeting with her sleeves. After the party, he entertained the idea of asking her himself about her behavior, but decided to ask Trey if he had any idea after dealing with some rule breakers. Trey couldn't exactly pinpoint anything in particular, he knew she mentioned in passing that she finds certain textures weird or wanders into a quiet location because she says 'I'm a little overwhelmed.'

After his overblot, Cater mentioned in passing how MC had no sense of danger around him, and literally approached him like normal. Everyone, even Crowley, was baffled to her behavior. "She even squished your cheeks and giggled because your skin is soft?" Riddle vaguely remembered her doing that, and the absolute confusion running through his head at her lack of fear.

When he finally asked her about it, MC replied with, "Oh, I'm on the autism spectrum. Some of my behavior might be weird, and I don't know if there's any documentation of autism in this world." After she said that, all of her behavior made sense to Riddle. He even began documenting her behaviors when he could, actions she does to calm down (aka stims), and things like her special interests. He wants to make sure he can understand her, and maybe help her advocate for herself.

Trey Clover
This man's pretty chill. He notices her behavior pretty quickly. He has a little sister, and he knows certain behaviors aren't normal. However, because his sister likely isn't as old as MC, he has to talk with Cater to see if any of her behavior could be considered "normal". When Cater confirms that he never seen similar behavior in his own sisters ("Then again," Cater chuckles, "not all women are the same.").

When he asked Ace, Deuce, Grim and MC to collect chestnuts to make mont blanc he noticed how she didn't really care, but she said she kind of wanted to stretch her legs anyways.

When the five made the mont blanc, Trey noticed that MC didn't eat much of it since she said she wasn't a big fan of the texture and wasn't really hungry, and gave the rest to Grim. He kept note of it, but didn't think of asking her.

Later, when the five of them and Crowley were in the library after the events of the Unbirthday Party the day before, Trey noticed she went missing and started to panic. A little while later MC came back with a book that caught her eye. He and Crowley had a word with her to tell them next time when she's going somewhere so they don't panic again.

Out of the five dudes of Heartslabyul, he was the last to find out that MC was on the spectrum when the six of them ate Riddle's tart. She said something along the lines of, "Oyster sauce can't change the texture, but it'll make it too salty for me. Sensory inputs, y'know?" Poor dude was so confused when Cater broke the news to him, but Trey is understanding since Cater himself doesn't like certain kinds of flavors.

He might even ask MC what her favorite desserts are and try to make them for her when he has the chance.

Cater Diamond
This dude's pretty easygoing, so he might be the most understanding out of everyone in Heartslabyul. When he first met MC he noticed how she paced around behind Ace and Deuce. When he asked them, Ace replied with, "Oh, she does that a lot. Says she has too much energy and has to use it somehow." He suggested that the three help him paint the roses red. They agreed to do so before class began (since Ace was wearing the collar and MC doesn't have magic, they had to use a paintbrush).

After Cater demonstrated how to paint the roses, he noticed that MC mimicked his actions exactly, down to the smallest movement. He found this interesting, even told a few of his classmates and Trey. Cater wanted to get to know her more, so he decided to talk with MC during lunch.

When he approached her, he noticed that she was somewhat shy and hardly talked much. Then again, she was eating so she likely didn't want to talk while eating food. After asking Deuce, he found out she's not exactly a talkative person.

Sometimes he noticed that she'd go into the light music room when nobody was there to study or read in peace. Part of him wanted to say hello, but he decided to respect the fact that she likely wanted some time alone and left.

When Cater came by after Trey, Ace, Deuce, Grim and MC finished making mont blanc he noticed that she didn't eat any (or had a tiny bit before giving it to Grim) because she didn't exactly like the texture. This made something click that something might be a little different with her. He decided to do some research, but couldn't find anything concrete.

During Riddle's overblot, he was shocked at MC's lack of a sense of danger and how she casually approached him and squished his cheeks and giggled uncontrollably. After the fight, she had Riddle's head resting in her lap when he asked MC about herself.

"Oh, I'm on the autism spectrum. I don't know if there's much documentation of it in this world, I hope my answer helps explain some of my behavior." this clicked with Cater, causing everything he noticed that was unique about her to finally make sense. When he finds out her special interest (let's just say it's drawing since it's one of mine), he might ask to take pictures of her with her art and post it on his Magicam account.

Deuce Spade (I basically gave up here)
This confused baby...he's trying his best. He was confused when MC would randomly start crying at first, he'll try to comfort her. Sometimes he sees her spinning around or walking in circles during PE, but doesn't think of asking her about it.

When Deuce and MC went to Sam's Shop to get ingredients for Trey, he noticed how she would often glance at random objects for a moment and then focus on another. Confused him, but didn't think of asking about it.

When he had the impromptu sleepover with Ace, Grim and MC he noticed how she could ramble on and on about drawing. When he asked how she could go on about that topic and seemingly not stop Ace broke the news to him.

Now he just has more understanding of her behavior, he didn't really change much when he found out MC was autistic (other than wondering why she wanted to draw his magical wheel).

Ace Trappola
This dude was pretty much the first to figure it out. When he and Grim had a quarrel on Main Street she was getting tears in her eyes randomly trying to stop everything from escalating.

Another time was when she randomly started crying in flying class, when he and Deuce asked her what was wrong she said between sniffles that sometimes she gets this urge to cry for no reason whatsoever, sometimes the same happens but she gets laughing fits.

He was the first one to find out MC is autistic when he goes to Ramshackle Dorm after he got his head 'cut off' by Riddle when she said she admired how he found advocating for himself so easily. When he asked her why, she replied with, "As someone on the autism spectrum I struggle with social skills, one of them being self advocacy."

After that, Ace tries his best to help her speak up for herself and comfort her if she randomly starts crying during class.