You can choose from six Nightfarer classes when you begin your Nightreign adventure, but the Duchess and Revenant are hidden behind some easy steps.

Here's what you need to do to unlock Revenant as a playable character in Nightreign: Obtain the Old Pocketwatch by attempting four runs, or beating Gladius once. Give the Old Pocketwatch to the Priestess to unlock the Duchess. Buy the Besmirched Frame from the Small Jar Bazaar for 1,500 Murk.

Defeating Tricepalos earns you the Blade of Night Fragment key item. Give it to the Guardian once you return to Rountable Hold, and complete the Remembrance. You get the Small Makeup Brush as a reward, a blue relic that gives the Revenant a power boost when her summoned spirits are on the field and increases Rune acquisition for the entire team.