Okay as you might've seen in my profile's bio I'm a httyd fan so this is why I'd cast who I put.
I know the person I chose would have to be doing voice acting for Ro or just major cgi so this first one isn't based on looks.
But I'd choose the lovely Kristen Wiig, because imagine Ruffnuts voice for Ro, that's how I see it in my head.

This second one isnt based off of looks either, it is based off of vibes. Now for the actual actor Maisie Williams.

Something else is if you look her up she might change her hair just as much as Ro lol