BOTH PLAYERS NEED AT LEAST 1 CHARACTER TO START - You can get characters by opening boxes. This costs gold, which you get from playing. The brown box gives 1 character. The purple box gives 3 characters. - Choose the character you want to play with (Don't click the arrows, use the keys mentions underneath the arrows). Then press play to play. - You can press "R" in a game to go back to the menu. - Player 1: A and D to move. W to jump. S to attack. - Player 2: Arrow keys to move. Up arrow to jump. Down arrow to attack. I've changed the characters, but the changes only show up in battle. Sorry that the Music Dragon glitches and appears as the Eater Dragon for P1.
Thanks to @Donsje_Partner!