Juvia is very obviously in love with her fellow Fairy Tail member Gray Fullbuster. She fell in love with him instantaneously during the Phantom attacks against the Fairy Tail guild. And although we all know she's nuts (It's hard to deny this), I've lately been trying to determine whether I could categorize her as a yandere or not. It's true that Juvia has shown signs of stalking her love interest, and even at one point attempted to give him a love potion to make him fall for her, but other than that, Juvia doesn't really show any signs of being a Yandere. Another category she shows signs of is that of a Dandere. She's super sweet to Gray, however she lacks the shyness ulterior of a Dandere. So, in my opinion, Juvia is probably 75% Yandere and 25% Dandere.

~~~Example~~~

Juvia: (Is hiding behind a bush watching Gray play beach volley ball with Natsu and the rest of their friends) ohh~ Gray Sama~ if only you would wrap Juvia in your big strong arms and love me~

Gray: (Strikes a hot pose as he serves the volley ball)

Juvia: oh, Gray Sama~! (Said in an annoying, high pitched, squeaky voice)

Lucy: Wow! Nice hit Gray! (Smiles and hits the ball back to him)

Juvia: (Growls in frustration) LOVE RIVAL!!

~~~End of Example~~~

So, as I have just demonstrated, Juvia has a lot of yandere characteristics with a touch of Dandere. So, yeah.. that's my conclusion.