She's a drama limiter on the show, since she's the most powerful and has a variety of weird spells she can stop or reverse any of the logical negative consequences of the big cool dragon action stuff, which means you can have those for spectacle without worrying about getting in the way of other things you want. So she serves a narrative purpose.
Aside that, she is mostly just gag boobs and the general archetype of the older sisterly side character, but who knows, she might get something more interesting. The anime seems to have ramped up the idea of hidden darker tones/history behind the dragons, with Tohru's vietnam flashbacks and etc. I wouldn't be surprised if she brought out some more serious side to her at some point in the manga's future, too, though I don't expect it any time soon.
I hope we get to see her and Fafnir's true forms as more than silhouettes at some point.