Topic: Hyrule Warriors - How does Co-Op Work?

I know that co-op in HW is one person on one screen, one person on another. But I'm really curious on what modes are there co-op in? Could you do Co-op in Adventure Mode or the main story?

You can co-op in any mode I think. It's available in Legend Mode (main story) , Free Mode, and Adventure Mode at least.
Just press + on a second controller when choosing a character.

I don't think you can co-op the first level in Legend Mode. And you only get access to Adventure, Free and Challenge modes after you beat the 1st level in Legend Mode. It took my 1 son about 18 minutes to beat that first level, his brother was not waiting very patiently Once that first level was beaten co-op was just a matter of pushing the + button. My kid says Adventure mode may be 1 player, but don't quote me on that.

I haven't tried co-op in adventure mode, but the game did have a splash screen telling me that every game mode could be played with two people.
The visual drop was quite painful for about 5 minutes after that first solo level but working with a friend to dominate the battlefield is fantastic fun.

I have played coop with my sister in adventure mode. You can play coop in every mode once you have beaten the first level in the story mode, as rjejr already explained.

As @rjejr stated, you unlock Co-Op after beating the first chapter of the story. Then, a second player can join in during character selection by pressing + on any other controller. Co-op works in all modes, unless a mission in Adventure Mode is explicitly limited to 1 character.

Player 1 uses the GamePad, while player 2 looks at the TV screen. Graphics resolution is reduced, and the game has more slowdons. Also, enemies that are indicated on the minimap take much longer to load, making grunts appear out of nowhere a staple.

Alright thanks everyone!