Dr.livesey is afraid of who?

Livesay says he fears Trelawney because he is not good at keeping secrets.

Trelawney," said the doctor, "I'll go with you; and I'll go bail for it, so will Jim, and be a credit to the undertaking. There's only one man I'm afraid of."

"And who's that?" cried the squire. "Name the dog, sir!"

"You," replied the doctor; "for you cannot hold your tongue."