Right then, Vivianna.
I think the second situation would be more hilarious. Vivianna could be fascinated by the invisible person Erika keeps talking to, and determined to get to the bottom of the matter or randomly decide she's now her best friend and insist on hugging her at odd intervals, or anything else which strikes your fancy really.
Not enemy, because I think one is enough and that would be....
Henri. Erika is wierd yes, and not the most popular around, but she's not timid, and I could really see these two clashing. Henri could be Rika's own hound from hell, and guess what, they're in the same House to top the cake. Of course, while all this is going on, Rika could also accidentally become Henri's first real friend at Hogwarts or something along that line, but that's up to you.
Wally hmm.....I can imagine her accidentally thinking that Rika's on drugs already, but obviously moral-bound Erika has to freak out at the suggestion. Definitely future friends.
As for Penelope.....oh joy oh joy, Erika can finally be the freaker instead of freaking out! (and yes, as of now, 'freaker' is officially a word
)