Hmm, and since I was rude enough not to list the rest of my characters with Isabella;

First of all there's James Potter here. I think he'd get along with her relatively well, in fact, and he could be a rival prankster along with Sirius as well as a past or present snog or date.. Nothing overly significant, given that another redhead forever holds his heart, but if nothing else his semi-player attitude dictates that he should have had his fair share of girls about the castle.

And she might be friend with Lydia. Who dated his best buddy. Which is to say that it wouldn't be too farfetched for them to have met each other through that channel, should Lydia and Colleen be friends.

That aside I have Charles Montague, Slytherin head boy, who I doubt would have little to do with her - unless he would have busted her for some manner of mischief at some point or another. I also have Garrett Abbott, Seventh Year Hufflepuff, but again I dont know how they'd know each other. Either are available for plottage, however.
So Self-Aware, So Full of Shit..