Preprocess function for body field?
Instead of using a preprocess function, you could accomplish the desired behavior using CSS.

Using the ::before and ::after selectors you can insert additional content. This content could be the textual " character (i.e., content: '"';) or you could display it as an (inline-)block, provide width, height, margin and padding and set a background image.

For example:

.field--name-body {
  padding-left: 1em;
  padding-right: 1em;
}

.field--name-body::before {
  content: '"';
  position: absolute;
  top: 0;
  left: 0;
}

.field--name-body::after {
  content: '"';
  position: absolute;
  right: 0;
  bottom: 0;
}


This creates some space around the body field and provides " characters at the top left and bottom right location.