Ember Boy and Ripple Girl

Ember Boy and Ripple Girl is a puzzle platformer where fire and water each pass through what stops the other. You switch between them to hold switches, move platforms and cross hazards, and almost every room needs both characters standing in the right place before either can go on — so the puzzle is sequencing rather than dexterity. Played alone it is a game about remembering where you left the other one. Played with somebody on the second set of keys, it becomes a conversation, which is clearly what it was built for.