Kateřina je zvláštní děvče :P
Why can't I use holka here?
You certainly can. If a sentence you tried was rejected, you have to tell us your complete answer, otherwise no-one can help you. If you are almost sure your answer is supposed to be correct, you should press "My answer should be accepted".
If we do use holka here for girl and not děvče, does "zvláštní" change at all? Or does this adjective always end in the í?
It follows the soft declension pattern so it is always -í in nominative. Check the Tips and notes for more.
What is the difference between holka and děvcě?
I understand either is acceptable here, but is there any time that one would be preferable over the other?