Terezi? She would be very, very, VERY happy about it. And then probably roleplay with Nep and scare the shit out of Vriska and Karkat یا something. I would probably just be freaked a bit at first and then probably we'd go on an adventure یا something.
If Megurine Luka were to be a dragon... Dang, her singing would be quite low pitched! Eh, she'd just sound like a slightly feminine Kaito. I'm not sure what she'd actually do. She'd probably continue singing.