How did Castle get his nickname? Why is he given that nickname by other people? Explain.

Castle gets his nickname because he runs so fast that he is like a ghost disappearing into the darkness of the night, particularly the night he and his mother ran from his father. Other people use the name "ghost" because Castle insists upon it..... it's how he introduces himself.

“Castle Cranshaw,” I said, then quickly clarified, “But everybody calls me Ghost.” By everybody, I meant nobody except me. That was my self-given nickname. Well, halfway self-given. The night me and Ma busted into Mr. Charles’s store, Mr. Charles looked at us like he was looking at two ghosts.