Lala

A currently unnamed Dullahan revealed at the end of Chapter 22. Originally implied to be the second "D", her discussions reveal she's merely there to warn Kurusu Kimihito of his impending death, leaving the question of who "D" is unanswered.

She is a Dullahan who claims to be an agent of Death.

Plot
After revealing her self to Kimihito and the others following a search for her lost head, she reveals that her species serves as form of Grim Reaper for people, arriving as a prelude to death, and in fact is perhaps, serious, that Kimihito's eventual marriage to any of his guest may result in his subsequent death.

Trivia

 * Fans theorized that "D" may be, in fact, a stalker of Kimito.
 * Another peculiar thing is the wording of the first letter itself. For some reason, "D" simply says "Marry Someone" as opposed to "Marry a Monster Girl" or something with similar connotations. This suggests that "D" simply has a problem with Kimito getting married and perhaps isn't a radical opposed to Cross-Species marriage. This helps support fan speculation that "D" may be a stalker.
 * After it was revealed that Doppel only wrote the first letter, and not the second, more poetic one, lends credence to this theory.
 * As of chapter 23, it has been revealed that the Dullahan is not the writer of the more poetic message.