Ransome is the cabin boy of the Covenant. He appears to be somewhat developmentally disabled, a condition made worse by the alcohol continually being pushed on him by second mate Mr. Riach. He remains deeply loyal to Mr. Shuan, the first mate, even though Mr. Shuan beats him regularly. Ransome has a very confused view of the world and appears to regard dry land as endlessly dangerous. He is also the one who introduces Davie to the idea of "twenty-pounders" (5.31), individuals transported to the Americas as indentured servants. This is supposed to be Davie's fate, according to Captain Hoseason (although luckily for Davie it doesn't work out that way).
Besides being something of a friend to Davie in those early days on the Covenant, Ransome's primary role in the book is, sadly, to be murdered. He is killed by a stray blow to the head from Mr. Shuan. It's this evidence of the desperate nature of the officers of the Covenant that leads Davie to warn Alan about Captain Hoseason's plot against him, which brings the two together in the first place.