It is used as a counterattack to redirect damage and knockback at the attacker when the user is hit. In addition, the move serves as an excellent edgeguarding tool against opponents with predictable recoveries.
Counter is based on a core element of Fire Emblem gameplay: in all games of the series, when a unit attacks another unit, the targeted unit will perform a counter-attack to strike back at their attacker if they currently have an equipped weapon with a compatible range.
Additionally, Fire Emblem: Path of Radiance introduced the identically-named Counter skill, which reflects any damage dealt to the skill's user back at the enemy. The skill does not exist in Marth or Roy's native games, however, and their appearance in Melee predates its introduction. In both of Ike's games the Counter skill can be assigned to him through the use of the Counter scroll item. Through re-classing to the Warrior class, Marth, Roy and Ike can all obtain the Counter skill as bonus units in Fire Emblem Awakening; however, Lucina is unable to obtain Counter under any circumstances, as it is exclusive to male units, and her father, Chrom, cannot pass it down to her either, both because he doesn't have access to the Warrior class and because Aether is the only skill he can pass down to his daughters even if he did.
Counter is Marth and Roy's down special move in Super Smash Bros. Melee. Marth or Roy will strike a pose with their sword in reverse grip and a small, metallic "chink" sound will be heard. For about a second, any attack other than a grab which the character using Counter is struck by will turn them invincible for the duration of their retaliating swing. If the attack is a projectile (such as Samus' Missile), it will simply be deflected behind them at about a 30° angle. It is worth noting that explosive projectiles can still explode and will still do damage if they are struck when the invincibility ceases.
Marth's countering stance has him face away while holding the Falchion slanted downward in reverse grip, and counterattacks with an outward slash in normal grip (akin to a composite version of his first input neutral attack and dash attack). His Counter starts-up faster and lasts twice as long as Roy's Counter, but deals a flat 7% damage regardless of the attack. Upon a successful counterattack, he might say "そこだ！" ("There!"), "見切った！" ("I've read you!") or "させるものか！" ("I won't let you!").
Roy's counterattack, on the other hand, has him rear down a bit as he holds the Sword of Seals in front, reverse grip for his countering stance, and counterattacks with an inward slash while still holding in reverse grip. His Counter deals x1.5 the original attack's damage and deals more knockback when striking with the full blade, but has slightly slower start-up and lasts for a shorter time, which makes it unable to be used for immediate countering, but is much more damaging compared to Marth's Counter. If the attack is a smash attack, the attacker is knocked back at a 90° angle. Since only damage is countered, and not knockback, some attacks will deal less knockback when countered, such as the forward smash of a Home-Run Bat. This applies to future games' counterattacks. If used in conjunction with Mr. Game & Watch's Oil Panic, it is the most powerful non-charged attack in Melee, delivering an OHKO (if Oil Panic has absorbed the highest possible amount of damage, which is 250% damage and also delivers an OHKO). His quotes are different from Marth's during a successful counterattack, either grunting or saying "いまだ!" ("Now!") or "そこ!" ("There!").
Marth's Counter has the same animation as in Melee. It has been generally buffed in terms of utility, now reflecting back x1.1 the original attack's damage, with a minimum damage of 8%, as opposed to a flat 7%, and allowing him to deliver a OHKO anyone close to him should he counterattack a strong enough hitbox. It retains its fast startup and long duration, but has lost the ability to deflect projectiles behind him. Curiously, it can be used on any vacuum created by an attack, such as Kirby's Inhale.
Ike's counterattack stance has him holds Ragnell right in front of himself in reverse grip, and he counterattacks with an inward slash while still holding it in reverse grip. Like Roy in Melee, Ike's Counter is not as fast as Marth's (the counterattack frames start 5 frames after Marth's), meaning it cannot be used for immediate countering, but the countering stance lasts for the same duration as Marth and has the same duration of counter frames, which gives it less vulnerability after the counter frames end due to its slightly slower activation. His counterattack has greater range and knockback than Marth, and reflects back x1.2 the original attack's damage, with the minimum damage being 10%. Upon a successful counterattack, he will shout "You're open!" In Japanese, he may say either "あまい!" ("Naive!") or "そこ!" ("There!").
In terms of aesthetic changes, all Counters have received a visual redesign that includes a wave like that of a heart monitor. This also applies to Corrin's, Palutena's, Little Mac's, Shulk's and Mii Swordfighter's respective counterattacks.
Marth's Counter once again has the same animation as in Melee and Brawl. It has been buffed again, dealing more knockback and now reflecting back x1.2 the original attack's damage. However, it can no longer be activated by vacuum attacks. In the game's release, there existed a bug where, if Marth were to use Counter just after being ledge trumped, he would propel backwards, which mostly resulted in him being KO'd. This glitch was patched in update 1.0.4. Interestingly enough, instead of using quotes for a successful counterattack, he now uses the generic grunts when he attacks. The reason for this change is unknown. His counterattacking quotes, however, can still be found in Sound Test.
As a clone of Marth, Lucina has the exact same Counter he does, the difference being the aesthetics: her countering stance mirrors her battle stance in Fire Emblem: Awakening, which is further referenced in her official artwork and her up taunt (though it instantly shifts to Marth's counter stance before she counters), and the counterattacking slash is in a more upward arc while her legs are not positioned on the ground during the active hitbox window. Before update 1.0.8, she used the generic grunts when he attacks upon a successful counterattack; after said update, she may say either "You're mine!" or "My turn!". In Japanese, she may say either "そこ!" ("There!") or "つきあり!" ("An opening!")
Ike's Counter has the same animation as in Brawl. It now activates slightly faster, now leaving him one frame less vulnerable at startup, and is the counterattack with the second longest counter window, behind Lucario's Double Team and Shulk's Vision when fresh.
Roy's Counter has a different animation: his countering stance is much different, now holding the Binding Blade in reverse grip while pointing it downward while standing upright. It has been buffed in the transition from Melee, gaining more counter frames, increased knockback, and range, but its damage multiplier was reduced to x1.35; this still leaves Roy's Counter with the highest damage multiplier (barring Shulk's Vision with Buster active, which has a x1.82 multiplier). Unlike Marth and Ike, his counterattacking quotes (and grunt) remain intact.
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Marth & Lucina
Like the other DLC characters, Roy has no custom move variations.
As an Echo Fighter of Roy, Chrom has the same Counter that he does, using the exact same stance as well. Also, Chrom shares Roy's x1.35 damage multiplier. Unlike Roy however, Chrom's counterattack does not have flame properties, and it has different knockback growth. Chrom’s counter will deal less knockback against weaker attacks, but more knockback against stronger attacks.