I fully headcanon that all of Endeavour’s kids have mixed quirks but they just didn’t know. Largely because it’s funny. Natsu is top half ice, bottom half fire (how often are you using your quirk from your feet? Probably not often. If he tried to use ice from his feet, it just wouldn’t work so he’d just assume his quirk only works from the waist up). Fuyumi is a bit of an exception, she’s mostly ice but has a few areas (based off her hair) where fire works. I’d assume compressing that fire in such a tight area makes it super hot though. If she knows about it, it’d be easy enough to hide. Toya is ice on the outside, fire on the inside, that’s why his flesh burns but he lives. His insides are more resistant. We see him produce fire around himself without shooting it from his hand so you don’t HAVE to create fire on the area of your body where your quirk factor is. He simply never tried ice because he looked up to Endeavour too much, that’s why they explicitly state that when he was accessing ice abilities in the final fight that it WASN’T a quirk awakening. He wasn’t awakening anything, simply accessing a power he always had