If you asked the members of class 1-A what Izukus's gender is, they would probably answer that he is a boy. Izuku has never objected to being called a boy, a son, and people using he/him pronouns for him.
If you asked Izuku he would probably shrug and say "A boy" because that's what he's always been. What else is there? He's heard the experience of being trans and non-binary described and they don't fit him. But then why does it not feel quite right. He's cis right? But he doubts most cis people have to chant "I'm not trans, I'm not trans" to themselves at night, But he can't tell anyone, then he just be doing it for attention.
Definitely cis.
Izuku never felt the need to hide that he's bi, he knew his mom would accept him as soon as he figured it out and didn't hesitate to tell her. He doesn't even really directly tell his friends, but the bi flag hanging in his bedroom and then dorm room was a strong indication.
Being so open about being part of the lgbtq community, it meant he was the first person Todoroki came to tell someone about his gender. Turns out he was actually a they. Izuku obviously accepts his friend, and for a while is the only one who knows.
He tells Todoroki a bit about his own gender issues.
"I feel mostly like a boy," Izuku says, "but like there is more, I don't know."
"A demiboy?" Todoroki suggests
"Maybe."
Izuku tried out different pronouns. Only online at first.
He tries out they/them pronouns, and they like it.
They try out she/her pronouns, and she likes it.
But sometimes she doesn't like it, and he only wants he/him.
They visit online trans spaces and learn about different identities.
He tries out the label bigender. He doesn't tell anyone else.
It doesn't fit.
Gen·der-flu·id: denoting or relating to a person who does not identify themselves as having a fixed gender.
It fits.
The first person he tells is Todoroki. Just as he accepted Todoroki, Todoroki accepts him.
Every day Todoroki asks them what their pronouns are for the day. Most days he shrugs and says "any." She doesn't understand how other genderfluid people just seem to know what their pronouns are for the day. She hopes she learns.
Eventually, they tell the rest of the self-proclaimed Dekusquad.
Of course, they are supportive. Nobody in the dekusquad is exactly straight anymore anyway.
Uraraka even makes her a heart-shaped genderfluid flag to pin to his bag. He sews it on so it won't get lost.
Some days aren't easy. One day he wakes up feeling pretty comfortably he/him and by the end of the day, she swears she wants nothing to do with he/him pronouns ever again.
It's confusing, and many days he wishes he could just be cis.
Just a boy.
But he isn't just a boy, just a son, or just he/him pronouns.
He's more.
