If you want to unlock Aero Rover and their weapon in Wuthering Waves, you'll have to go through the fourth act of Rinascita's main story.

It’s not that hard to find the best weapon for Aero Rover in Wuthering Waves. The game rewards players with Bloodpact's Pledge at the end of the "The Maiden, The Defier, The Death Crier" quest.