Ghost

In The Elder Scrolls V: Skyrim, ghosts look like transparent bluish NPCs. They may or may not be hostile, it all depends on the ghost.

Friendly ghosts

 * Atmah
 * Elvali Veren
 * Gallus Desidenius
 * Hafnar Ice-Fist
 * Headless Horseman
 * Helgi
 * Svaknir‏‎
 * Takes-In-Light
 * Valdar

Unfriendly ghosts

 * Subjugated ghosts, guarding Rannveig's Fast
 * Yngvild ghosts, guarding Arondil

Geist