Lionel Merble is Valencia Pilgrim's father and Billy's father-in-law. He is an optometrist who brings Billy in on his business, giving him the kind of wealth and opportunity Billy never had growing up. It's on a trip to an optometry conference with Lionel that Billy has the plain crash that fractures his skull and puts him in the hospital with Bertram Copeland Rumfoord. Lionel is killed instantly.
Lionel's only real significance in the book is his liking for barbershop quartets. He listens to one perform on the flight that kills him, the same quartet that sang at Billy's wedding anniversary party in Chapter 8. This barbershop quartet makes Billy feel sick and unsettled because the four open-mouthed men remind him of his four German guards in Dresden.