I

As spoken by the Tenth Doctor and the entity possessing Sky Silvestry, I (Morse code: .. (TV: The Man Who Never Was)) was the ninth letter of the alphabet, following H and preceding J. (TV: Midnight)

Astonished at the sight of the Fifth Doctor regenerating, Peri Brown stuttered and said "three Is in one breath", to which the newly regenerated Sixth Doctor remarked made her sound like a "rather egotistical young lady." (TV: The Caves of Androzani)

It was this letter which distinguished "Linda with an I" from fellow Big Brother housemate Lynda Moss, or "Lynda with a Y." (TV: Bad Wolf)