E

As spoken by Sarah Jane Smith, E was the fifth letter of the alphabet, following D and preceeding F. (TV: The Masque of Mandragora)

The letter E appeared in the word dangle but not in the word gland. (TV: Bad Wolf)

While unable to directly oppose Ruby White, Mr Smith told Clyde Langer "letter two (B), letter five (E), letter twenty three (W)", which Clyde interpreted as "beware". Mr Smith then added "letter eighteen (R)", with Clyde deducing that he was trying to warn him about Ruby. (TV: Goodbye, Sarah Jane Smith)