Beat the game and collect 151 stars. Then, play each of the four Caesar vs. Outlaw missions. The missions must be played in order. In each mission, there is a tile with a statue. Move Caesar to this tile to view some dialogue and collect an orb. Afterward, finish the mission. Once you have finished all four missions with the orbs, Mercival will be unlocked.
Play as Elodie
|Unlockable||How to Unlock|
|Elodie (Arcade, Multiplayer)||Beat mission 2, act 7|