I love Roy being 1/4 Ice Dragon (if his mom is Ninian, which seems to be a popular route) so much, so let's talk headcanons
The Binding Blade, due to specifically being made to target dragons, actually kind of hurts Roy whenever he uses it - it’s like a dull burning in his hands. He usually tries to ignore it, but it’s still one of the reasons he wears fingerless gloves
Physically there’s only a few dragonic traits: his ears aren’t pointed, but he does have small fangs and his nails grow real sharp real fast. He's kind of shy about them actually - he'll do things like cover his mouth if he's laughing too much or smiling too wide, and bite at his nails a lot
As far as he’s aware, he can’t transform at all. It makes him upset for more reasons than just the literal “cool factor”... he doesn’t know much about his mom aside from stories, and as an empathetic person who learns by experience and putting himself in others’ shoes he would’ve wanted to transform so he could grown closer to her in that way
One day, Robin and Corrin find out when they jokingly include Roy in their Manakete discussion and he completely overreacts
“Come on Corrin, why don’t you ask someone who actually counts as a dragon, like Zard or I dunno...” *looks around the room for the nearest person* “Roy? Heh-” *Roy drops his cup of water, face pale* “WHA- HOW DID YOU K N O W” “...Excuse me”
The two of them - especially a very excited Corrin - teach him about various aspects about Manaketes, including the different kinds of Dragonstones. Eventually they get him an Icestone and have him try it out...
And while he doesn’t fully transform, he partially does!! He gets the fins and tail of an Ice Dragon, the tips of his hair turn ice blue and he gets a bit of ice breath too!!!
Roy is absolutely ecstatic about this. He goes around shooting ice everywhere and people are super confused, and he brags about having both ice and fire abilities (though he can’t show them off simultaneously because yowch why does the Binding Blade hurt even more), and he even learns how to make snowcones fjdhfjdfj... and he feels a lot closer to Ninian too :>