Shadow

Shadow is a mysterious figure appearing to Nightingale as a horse, willing to aid her during the 13 Days.

13 Days
Shadow is Nightingale’s trusty steed, but his origin and motives are unknown. He always awaits her call and takes every order of hers, but all she asks of him is to get her to her destination. Shadow is able to defend himself and speak to Nightingale through a telepathic link.

A Lost Fantasy
He returns in A Lost Fantasy after somehow surviving the end of the 13 Days. He still follows Nightingale to battle, and even boasts a new form. He can transform between the form of a horse and Winged Xalyts, a dragon-like beast native to Pandora. However, Shadow is unable to speak in that form.

Trivia

 * He resembles Famine’s black horse from the biblical tales of the Four Horsemen of the Apocalypse.
 * Nightingale's connection to Shadow is similar to the Four Horseman of The Apocalpyse. She shares a connection to the horse, and rides him to nearly every battle during the 13 Days.