If i keep a cashier's check that was sent to me in my name is it illegal and what are the laws?

a girl sent it to me and asks that i take some of it and give her the rest of it back. fyi it's her money. but i wanna keep the whole thing. is there anything she can do to stop me or anything she can do if i have already cashed it?
Update: she lives in ohio and i live tn. does that matter?
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