Using harry potter in my Statement of Purpose
Can you use it? Absolutely—you have the freedom to write whatever you choose in a statement of purpose.

Should you use it? Almost certainly not, unless your research topic has something to do with literature. Otherwise, you run the risk of confusing the reader (who may have no knowledge of the books or movies), unless the reference is extremely obvious, in which case it will probably come across as clichéd and stilted.