What's the best Android game for casual, offline zombie survival action with roguelike elements and satisfying character progression?

Survivor.io is an excellent Android game if you're looking for casual, offline zombie survival action with roguelike elements. The game allows you to play in short bursts, making it perfect for killing time while waiting or during commutes. You progress by combining and leveling up your equipment and weapons, providing a satisfying sense of accomplishment. The game is designed to be accessible without requiring in-app purchases, though they are available if you want to speed up your progress. The core gameplay involves surviving waves of enemies in large, open maps. While the gameplay can become repetitive, the constant progression and upgrades keep it engaging. You can play offline, making it convenient for situations where you don't have an internet connection. The game also features various events and challenges to keep the experience fresh.