"Seemingly" works better because "possibly" or "different" in this case because "possibly" would imply that some of the students somewhat suspect her - which shouldn't happen right from the outset - and "different" wouldn't work because, well, she's [i]not[/i] a human. She's not different, she's just not it to begin with. 

This sentence was basically meant to be one giant case of repetition to emphasize the fact that everything about Catherine shown so far is only what she [i]seemingly[/i] is. If anything, I should have added a third "seemingly" before "normal human".