How they met:
Not through the obvious hero course overlap or anything. I could see them actually properly clicking during a joint training exercise where Mina got paired with the general studies/support kids for a "work with people outside your usual circle" assignment. Shinso was blunt, unbothered by her chaos, and didn't flinch when she got weird with her acid in a drill. She decided on the spot he was "funny in a mean way" and adopted him.
Why they started talking:
Mina is incapable of leaving anyone alone. She clocked that Shinso mostly got left out of class 1-A group stuff even after he transferred, and made it her mission to loop him in w/ the baku squad sitting next to him uninvited, texting him memes at 1am, that kind of thing. He complained about it constantly buuuut, never actually told her to stop.
Why they're interested in the other:
Shinso finds her exhausting in the best way. she says what she's thinking, drags him into things he'd never do alone, and somehow makes him laugh without trying. Mina likes that he's not performing anything for her. He's dry and sarcastic and doesn't do the "everything's fine!" thing a lot of their friends do. he lets things be as messy as they are, and so does she.
Favorite holiday to spend together:
Halloween, no contest. Mina goes all out on a couple's costume every year (usually something horror-movie themed, sometimes something that makes Shinso deeply regret agreeing), and he pretends to hate it while clearly having a good time.
When they'd start dating:
Not until well after graduation, once they're both a couple years into hero work. There's a long stretch of "are they aren't they" that everyone around them can see except them. It probably takes Mina getting hurt on the job and Shinso showing up before anyone even called him for it to click for both of them.
Pet names or nah?
Mina absolutely loves the pet names, and they escalate specifically to annoy him ("babe" to "honeybug" to increasingly unhinged nicknames). Shinso never uses one seriously, but "idiot" said in a certain tone counts as one at this point and they both know it.
How long they last as a couple:
Long-term, low-drama once they actually get together. I bet they argue like normal people but nothing catastrophic. I'd say they're one of the "everyone assumed they'd get married eventually" couples.
Show they binge together:
Trashy competition reality TV, unironically. Cooking shows, dating shows, all of it. Mina live-commentates the whole thing and Shinso acts annoyed but has definitely picked a favorite contestant he won't admit to caring abou.