Two heroes, one temple
Fireboy & Watergirl 1: The Forest Temple is the first chapter of the puzzle-platformer series. You guide Fireboy and Watergirl through a 2D temple full of platforms, switches and traps, and each character moves and reacts differently to what's around them. The goal in each room is straightforward: solve the puzzle, reach the exit, beat the clock and grab as many diamonds as you can along the way. Rooms are built so that the two characters have to do different things at the same time, so you are always thinking about both of them rather than just one.
Playing together
You can play solo, controlling both characters yourself, or hand one of them to a friend. Local co-op runs on the same device, which suits a shared keyboard or controller setup. If your friend is somewhere else, Steam's Remote Play Together lets them join in from their own machine. Either way the two of you are working through the same rooms at the same time, with one person holding a platform down while the other climbs, or one hitting a switch so the other can pass. Coordination matters more than speed alone, and the timer keeps the pace moving.
Before you start
This one earns its reputation for being difficult. Expect to fail a room a few times before the layout clicks, and expect the clock to punish hesitation once you do understand it. The puzzles are the core of the game, so if you enjoy working out how two characters can be positioned to open a path, this fits. It's the start of the series, so it's the natural entry point if you've never played a Fireboy & Watergirl game before. Decide up front whether you're playing solo or with a friend, since that changes how you split attention across the two heroes.