I am sorry to everyone else here for the stupidly long post, but I think that ignorance should be met with a full factual rebuttal so that it can be stopped and prevented from spreading.
@smolbaka
So he was born with gender dysphoria? I've yet to see anything like that in this manga.
Chapter 22/23: If you have yet to see any indication at all of gender dysphoria in this manga I refuse to believe you ever read these two chapters, which have by far the most obvious and extreme presentation of gender dysphoria in the manga. Mogumo quite literally was holding a kitchen knife against their throat because their voice had started to drop. "My voice... is changing...?" "That's it... I should just gouge it out..." "If I take out my Adam's apple... I won't have to be a boy..." "IT'S YOUR FAULT FOR GIVING BIRTH TO ME LIKE THIS!!"
Chapter 23 came out three months ago, not long enough ago for you to have completely forgotten everything like one may do for the early chapters that came out 2 years ago. So maybe you just don't know what gender dysphoria looks like, which is understandable, it's very hard to tell unless you know what it is and the many ways it can affect people. But this isn't something that happened because Mogumo's dad didn't approve of how they dressed.
This is what gender dysphoria is. The fear over your voice changing, wanting to claw out your Adam's apple, the desperate hope that you don't have to continue growing as a boy, and then the spite and anger at how you weren't born the right way are feelings felt by most trans people who were assigned male at birth. This whole post is a list of things, but just this one bit should be enough to show on its own that Mogumo has gender dysphoria. Everything else is to further prove that point and to further prove that Mogumo is non-binary, rather than just being binary trans.
Everything you say just seems to indicate that you don't pay attention to what's going on in the story and that you don't care about the characters. Because if you did either of those things, you wouldn't so casually dismiss Mogumo's dysphoria as if it doesn't exist.
No indication of mental illness, or proof he has it. but alot of evidence of environmental influence on his identity. Especially at an early age.
As is clearly mentioned, at an early age Mogumo had a lot more freedom in what they could wear and how they could act. Their dad wasn't on their case much about making them conform, although he did say for Mogumo to stop saying they aren't a boy. Mogumo was allowed to explore themselves and their identity at an early age which is in part how they were able to understand themselves to be non-binary starting in middle school.
He liked girl clothes, wanted to try them on. Daddy doesn't like it. Family breaks apart due to daddy's narrowed view. It looks like mogumo just sorta stopped being a boy because of his dad. Rather than some brain chemical imbalance.
Again, you clearly haven't been reading the story very well if you think Mogumo's father's aggressiveness of rigid gender conformity came before Mogumo expressed themselves to not be a boy. That came first.
I did give you a source where the mangaka himself explicitly states that Mogumo is non-binary and that their character came to be after learning about, meeting with, and interviewing non-binary people. Here it is again, I recommend reading it so you can check out some of the mangaka's perspective. https://media.comicspace.jp/archives/16192
But if you want some more instances of gender dysphoria, we can also go through some. We can also go through more gender affirming things, as being trans (including being non-binary) is not just about the dysphoria.
Chapter 1: Mogumo uses the gender neutral bathroom, considering the entire rest of the manga this is kind of obviously due to being more comfortable with it than with either the male or female bathrooms.
Chapter 1: Mogumo's harsh recoil after being told the cafe is for crossdressing guys asking "Does that mean... that you are saying you want me to be a boy?", feeling disgust that they are being considered a boy.
Chapter 1: Mogumo saying they are not a boy. "I mean, you're a boy, right?" "...No, I'm not."
Chapter 1: Mogumo saying they never wanted to be a girl and affirming again that they are not a boy. "I never said I wanted to be a girl. And... I really am not a boy."
Chapter 1: Mogumo reaffirming that they are not a boy nor a girl. "I'm not a boy or a girl!!" This is followed with Mogumo saying "YOU GUYS SHOULDN'T BE MAKING UP YOUR MIND ABOUT MY GENDER!!" because people kept assuming them to be male or female. This also goes against part of your point of Mogumo being pushed to this because of their dad, Mogumo pretty clearly knows who they are and they aren't letting other people decide for them. This contradicts any ideas that Mogumo disassociates themself due to their dad's strict rules on gender conformity.
Chapter 1: Mogumo only agreeing to stay at the cafe due to being allowed to not be a boy.
Chapter 2: Mogumo feeling like they aren't the same sex as Tetsu, this feeling of not aligning with one's birth sex is a pretty direct on the nose feeling of dysphoria.
Chapter 2: Mogumo suggesting that they have run away from many situations where they weren't accepted as being non-binary.
Chapter 2: Mogumo expressing the huge relief they felt when being told they didn't have to be a boy, and that have always been told they to be a boy while growing up.
Chapter 3: Mogumo saying they aren't a girl but also don't want to be thought of as a boy either, and saying how them liking female clothing doesn't make them a girl and isn't the same as wanting to be a girl. Mogumo pretty well understands that there is a separation between gender and gender expression.
Chapter 4: Mogumo expressing more relief and saying that they can work at the cafe when Tetsu says that they are going to change policy so that people don't have to say that they are "girlyboys"
Chapters 1-4: Less Mogumo, but more a comparison to Mei. Mei throughout here is shown with her own dysphoria, however, she is in a position at that point where she hasn't come to terms with being trans. She gets jealous that Mogumo can assert that they are not a boy since that is something Mei has always wanted to do but felt that she couldn't due to the societal pressure around her. It's why she had latched on to the term "girlyboy" as she felt it was the closest she could be while being stuck as a boy. Mogumo on the other hand has already come to terms with their gender, which is why they are able to assert themselves and put their foot down that they are non-binary and not a boy or a girl. Some of these dysphoric feelings expressed by Mei here are later shown with Mogumo through flashbacks later on. However, something that is shown here is how Mogumo feels alone, out of place, depressed (expanded more later on in the story), and disconnected from people. While none of these mean gender dysphoria themselves, those feelings are very often a consequence of having gender dysphoria.
Chapter 5: Mogumo saying "watashi" and "atashi" a bit to themselves and clearly feeling some discomfort with them. Mogumo thought that because they like Tetsu and to them Tetsu hasn't said he's not straight that they should try to fit more as a girl, which immediately causes discomfort. For someone who is non-binary, dysphoria can come from imagining oneself as/being assigned/whatever as either binary gender, not just their birth one, which is important to note. When Mogumo feels uncomfortable being thought of as a girl throughout this manga, it's not an indication that they are just a boy, just an indication that they aren't a girl.
Chapter 6: More feelings of having to be a girl in order to be with Tetsu and they continue to cause discomfort for Mogumo.
Chapter 7: Mogumo more explicitly saying that they don't feel comfortable being that feminine and being a girl.
Chapter 7: Furthering this, Mogumo asks to where shorts instead of a short skirt for their summer uniform. Not enough to be masculine, but a step away from being too feminine.
Chapter 10: The first proper flashback. Mogumo, aged 7, once again, affirming that they are not a guy. However, unlike previously, Mogumo hasn't figured out their identity at this point, as Sakura calls Mogumo her sister. We can also see here when Mogumo's family was a bit more accepting of things, and allowed Mogumo to dress how they wanted to and say how they identify with more freedom. This again contradicts any claim that Mogumo thinks they aren't a boy because of their parents, it is quite clear that they dont identify as a boy because they already know they aren't one. This gets expanded upon later as the main reason their dad gets angry and huge problems in the family happen is due to Mogumo asserting that they are non-binary, not the other way around of them thinking they are non-binary because of their family.
Chapter 11: Mogumo saying how they felt good that Kotone stayed with them because it made them feel like it was proof that they weren't a guy. Mogumo is feeling a pretty great amount of sadness when asking "Does this mean that I'm a guy in the end...?" because they desperately don't want it to be true. Tetsu affirming that they aren't a guy and are just Mogumo brings Mogumo nearly to tears, and maybe all the way we don't see.
Chapter 11: Mogumo continues the feelings of discomfort that stem from how Kotone sees them and what Kotone wants them to be and how neither are what Mogumo is and what Mogumo wants to be. As Kotone seeing Mogumo as a guy has been growing over time and the Kotone's desire for Mogumo to embrace more femininity are both things Mogumo doesn't feel comfortable with.
Chapter 13: Mogumo leaves Tetsu meeting with his friends due to the pretty obvious discomfort of being referred to as Tetsu's girlfriend and then having it just played down as "Oh, it's just Mogumo" as that statement means they see Mogumo as a guy and due to being a guy also not being able to be with Tetsu.
Chapter 13: Mogumo getting mad that Tetsu's friends see them as a guy.
Chapter 13: Mogumo showing a whole lot of relief that Tetsu's friends end up being pretty accepting of them.
Chapter 14: Mogumo showing discomfort at not being sure whether or not Suzu's boyfriend is accepting of them, and is uncomfortable saying that they aren't a boy or a girl until Suzu gives them the okay. Not being sure whether or not to reveal to someone your identity causes a lot of discomfort for a whole lot of trans people, non-binary people even more so due to the even greater discrimination and lack of acceptance.
Chapter 14: Suzu's boyfriend saying that Mogumo not being a boy or a girl is very fitting of them. Mogumo feels relief at this and also wonders if more people will accept them and understand them if they open up about it more.
Chapter 14: Mogumo saying "I was only able to always be myself... because I wasn't alone." This whole scene is Mogumo pretty directly stating that the "myself" they are talking about is them being non-binary and that they have only been able to be so open about it lately because they have supportive people around them. Because their friends are accepting and supporting their identity, they are able to be more themself.
Chapter 16: Mogumo asserting to Kotone that they are not a girl, and that Kotone should stop using them as a replacement for one.
Chapter 18: A whole chapter full of discomfort while being repeatedly called "brother" and being asked to come home, a place where we know Mogumo's identity is very much not accepted.
Chapter 19: Mogumo feels relief when Sakura this time isn't antagonistic and instead helps pick out a cuter outfit. This happens again later in the chapter when Sakura picks out a cute hair tie for Mogumo. Throughout this chapter you can feel a weird emotion going through Mogumo has they are unsure exactly how things lie with Sakura. The unsettling feeling of going between calling them "bro", supporting them in the clothing they are wearing, and general uncertainty due to the family situation and lack of acceptance at home.
Chapter 20: Mogumo talking about Sakura as an ideal image.
Chapter 21: Sakura remarking that she thought they were a happy family "until Ryuu-chan ended up like that...", while saying earlier that she thought of Mogumo has her sister. This is pretty clearly stating that when Mogumo started saying they were non-binary and their gender dysphoria was becoming a whole lot more pronounced, things started breaking down.
Chapter 21: Sakura saying that "Ryuunosuke is the result of us accommodating to that stuff." This is talking about how the family was more accepting and understanding of Mogumo being whoever they wanted, and when they started not accommodating that stuff things broke down and Sakura also started taking heat for not being feminine enough. This is quite literally saying that Mogumo identifying as non-binary is the result of letting them express themselves how they want and letting them explore their identity and expression, and letting them be who they wanted to be. This is another big thing that goes against your point of being pressured into this identity by the lack of support, as this clearly was not the case. Mogumo was allowed to explore themselves, which is how they ended up identifying this way, not because they were just gender non-conforming and their dad made it seem like a boy couldn't be that.
Chapter 23: A bunch of further assertions from Mogumo in their very early childhood that they aren't a boy. Including one assertion when their mother says "There are plenty of gentle boys like you in the world," a statement pretty similar to how many people tell trans women and non binary people who were assigned male at birth that they can just be a femboy/feminine guy.
Chapter 23: Mogumo saying "It's amazing how everyone understands what gender they are." At this point back in elementary school, Mogumo still hasn't figured out their gender. At this point all they know is that they don't feel like a boy and that they like girl's clothes.
Chapter 23: Mogumo's dad is starting to be more aggressive about the gender conformity at this point starting around middle school. Mogumo is already realizing at this point that they don't fit in with either male or female.
Chapter 23: Mogumo explicitly says they weren't a boy but also that they didn't want to be a girl either. That they didn't know where they belonged. And that their whole body felt wrapped in discomfort. They wonder if everyone else feels this way or if it's just them. This is all dysphoria, these feelings are extremely common among people struggling with gender dysphoria.
Chapter 23: Their dad no longer being home much due to his job, puberty beginning, and the continued denial of their identity causes a great deal of stress for Mogumo and leads them to emotional outbursts and whatnot.
TL;DR
Mogumo has been experiencing gender dysphoria since early childhood. Mogumo is non-binary.