Hi, so I do not have any specific person that I would give as a picture for how Cassie looks like. So I have given some of her Physical and mental qualities (there might be more. You can discover them as the story evolves) here: 

Physical:                                                                                                                                                                         

+ Height maybe about 5'5" (165 cm)

+ Black Hair past the shoulders

+ Dark Brown eyes

Mental Qualities:

+ Determined

+ Can find good and bad in everyone. (Maybe even in Umbridge)

+ Smart

+ Kind and Loyal

+ Fights for what she thinks is right.

+ Tells lame jokes.

+ Dual Personality (Angel in front of teachers and clown/devil in front of friends)

Hopefully it helped you to imagine/get an idea on how Cassie is. Any other doubts or corrections, please feel free to comment it here. 

Thank you so much for checking out my story and have a fun journey ahead. 

~C.G