Marvin Prune is a golden ticket finder in Willy Wonka's factory.
A (Literally) Missing Child
In 1972, Roald Dahl sent Marvin Prune's exit to The Horn Book Magazine, hoping they would publish it as a 'missing chapter'. Instead, the magazine printed a work by Elanor Cameron, which almost seemed like an assault on Roald himself, many fans say. Roald never got the chapter back, and its been missing ever since.
However, there is an earlier version of Marvin Prune that exists. None of the children (including Marvin) at this point had distinct characteristics. He was supposedly made into "Children's-Delight" (the precursor of Spotty Powder). However, towards the end of the draft, Mr. Wonka reassured Charlie that none of the children were seriously injured (RD 2/7/2, pages 36A-43, 83).
Marvin's family is probably made up of himself and his parents, among others.
You can find a fan-made version of Marvin's exit at , made by a creator called Jimmy-C-Lombardo.
1. See DeviantArt link
2. Inside Charlie's Chocolate Factory, By Lucy Mangan, published by Penguin Books.