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.
In Super Smash Bros. Melee, 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!").
Marth's Counter in Brawl has the same animation as in Melee. It has been generally buffed in terms of utility, now reflecting back 1.1× of 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 (such as a thrown Bob-omb, which deals 40%). 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.
In Super Smash Bros. 4 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 the Sound Test.
In Super Smash Bros. Ultimate, Marth's Counter retains the same stance and attack animation as in SSB4. However, it no longer has him merely use his standard attack grunts during the actual counterattack, as his counterattack quotes from Brawl have returned with new voice clips; in particular, he now has English-language quotes for successfully countering an attack, uttering "My turn!", "Predictable!" or "Nice try!" when doing so. Both Counter and the counterattack have increased ending lag, making the move easier to punish if whiffed, and limiting its utility in edgeguarding, respectively; however, the counterattack has faster startup.
As a clone of Marth, Lucina has the same Counter that he does, with the same 1.2× multiplier as well. The main difference setting it apart from Marth's Counter is the animation, as 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 she is kneeling during the active hitbox window; the altered animation for Lucina's Counter causes it to have noticeably shorter horizontal range in front of her than Marth's Counter. Before update 1.0.8 in SSB4, she used the generic grunts when she attacks upon a successful counterattack; after said update (as well as in Ultimate), 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!"). In Ultimate, Lucina's Counter shares the changes made to Marth's version.
Roy's counterattack in Super Smash Bros. Melee has him rear down a bit as he holds the Binding Blade 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!").
In Super Smash Bros. 4, 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.
In Ultimate, Roy's Counter retains its stance and attack animation from the previous installment. Additionally, Roy's lines are now dubbed into English, as he may utter "Now!" or "Gotcha!" when successfully countering an attack. Roy's Counter has a longer duration, and the counterattack has increased knockback that makes it even stronger.
As an Echo Fighter of Roy, Chrom has the same Counter that he does, using the exact same stance and counterattack animation as well. Also, Chrom shares Roy's x1.35 damage multiplier on his counterattack. Unlike Roy however, Chrom's counterattack does not have flame properties, and it has lower base knockback but even higher knockback scaling, making it weaker against weaker attacks, but stronger against more powerful ones. When successfully countering an attack, Chrom may utter "No way!" or "Not today!" in English.
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's, and reflects back x1.2 the original attack's damage, with the minimum damage being 10%. It gains OHKO power countering at least 35% damage (such as a Bob-omb). 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 Super Smash Bros. 4, 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.
In Super Smash Bros. Ultimate, Ike's Counter behaves similarly to in previous iterations, but now has a blue flame effect. Additionally, Ike has new quotes when successfully performing a counterattack; in English, Ike's Path of Radiance version says "Gotcha!", while his Radiant Dawn version says "Amateur!"
Special Move customization was added in Super Smash Bros. 4. These are the variations:
Like the other DLC characters, Roy has no custom move variations.
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 returns an amount of damage dealt to the skill's user to 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.