The idea here is two life long friends. Your character has been the ultimate tomboy, no stereotypicaly girly hobbies/possessions, short hair, gender neutral name, dresses like a boy other than underwear. As kids, there was no way to know she was a girl without either her saying so, or being seen naked, or going somewhere like a pool/water park, where the swimsuit would give it away. My character just always assumed she was a boy and your character either didn't care or was too nervous/embarrassed to admit to being a girl.
Of course this was a secret that time could not hide forever. Her breasts have started to grow, though they are still small enough she can hide them with effort, as well as any other changes coming with puberty that make it more obvious she is a girl. Perhaps she is developing a crush and/or lust for him too.
Either way, somehow the truth of her gender is somehow finally revealed. Maybe he walks in on her mid-changing, maybe a class water park/pool field trip/shared gym class in the school pool or something else we can decide on.
Once the secret is out, they both try to continue as things were, but it is more awkward with the truth out there and both being aware they are boy and girl. There becomes a blurry line between the stuff they usually do and a sort of casual dating, before too long, they start talking about sex and agree to try and keep doing it after they start.
I would be playing a 13-14 year old boy, alongside a same age girl (it could maybe be slightly older but I think that is more realistic in the age the secret would come out and they would make this transition), exploring as they shift from pure friendship to a strange mix of friends, sex friends, and couple. A relationship they try to keep the same, but they can't prevent the change, though the changes are fairly subtle and gradual to the point they might not even always be aware of them and whether or not the girl remains quite so tomboy, or decides to 'feminize' at least somewhat under the circumstances.