Houkai Gakuen, also known as Guns Girl, is a popular Chinese mobile game series developed by miHoYo. The series is set in a post-apocalyptic world ravaged by an energy known as the 'Houkai' that transforms creatures into monstrous beings. Players control Valkyries, girls with special abilities, to fight against these creatures. The game features a mix of action, strategy, and RPG elements, with an engaging storyline and diverse characters.