Counter is a special attack that can be used by both Marth and Roy in Super Smash Bros. Melee, as well as a special move in Super Smash Bros. Brawl used by Marth and Ike. The attack is performed by tilting the Control Stick downwards, and then pressing the B Button. Marth or Roy will crouch down, and hold their sword at the side of their head. Once an opponent comes to attack, they will miss, and Marth or Roy will automatically attack the foe. The attack must be used quickly, as it won't work if it is kept out for too long. This is the main reason the attack is hard to master, let alone use. However, the attack is very useful once done right.
Interestingly, when Marth uses Counter, he will say "Soko da", which translates to "There it is". This means he has found an opening to attack, and there it is.
Lucario's move, Double Team, is very similar to this move, the difference being the damage being affected by Lucario's damage percentage.