Inactive or Dead Ghosts can be found throughout the world. Guardians receive a small XP bonus for finding and reviving this lost Ghosts. Reviving Ghosts so far has only been tied to unlocking grimoire cards.
Over the course of the Guardian's story, they can find different shells to equip to their Ghosts. These Ghost Shells can be acquired, but at present the means are unknown. The only confirmed customization option so far is a pre-order bonus for a red shell.