A KO, short for knock-out or knock-off, commonly known as a death or a kill or even a finishing blow by Stamina Mode, is the term used to describe the situation where a player is launched beyond the stage's blast lines by the opponent's actions. The KO'd player is sent to a revival platform to re-enter the match. Losing all HP in Stamina Mode results in a KO without crossing a blast line, and some attacks and hazards can cause instant KOs without the target crossing the blast line or being launched at all.
All characters have the same KO explosion, namely a gigantic colorful blast that extends in the direction the victim was KO'd, in the color of the player's port: Player One's explosion is red, Player Two's is blue, CPUs are grey, and so on. Only two characters have slightly different KO explosions, Mega Man and Luma: Mega Man will split up into orbs the color of his costume (alongside the normal explosion), like he does when he dies in the Mega Man series (they will sometimes not appear if many Mega Mans are KO'd at the same time), while Luma's explosion is smaller and creates colorful stars.
Once KO'd, the following may then happen depending on the type of match:
In Super Smash Bros. and Melee, each character has a "KO property" which starts out empty. When characters are hit by any attack (be it a physical attack, a projectile, an item, etc.), their KO property is set to match whoever made the hit. Stage elements do not change the KO property. The KO property is reset once a character lands on the ground and is no longer moving as a result of the attack's knockback. When the character is KO'd, the KO goes to whoever matches the KO property. If the property is empty, the KO counts as a self-destruct. In simple terms, it goes to whoever made the last hit.
However, this format was altered for Super Smash Bros. Brawl. In Brawl, a character's KO property is only reset 3 minutes after landing on the ground. This essentially means that a character must refrain from being hit for 3 minutes for a self-destruct to actually count as such; otherwise a self-destruct will be credited as another player's KO. While this was possibly implemented to avoid griefing in online matches (by self-destructing simply to deprive other players of points), in reality its main effect is giving other players illegitimate points, as it's clear that jumping off the edge a few seconds after successfully recovering is not a KO.
There are exceptions, however, particularly during most Final Smashes. If players jump off the stage during a Final Smash such as Zero Laser, then, unless someone else hit them before landing, it will count towards the user of the Final Smash, instead of the last one to hit them (Samus in this case). Moreover, getting KO'd during the usage of certain Final Smashes such as Giga Bowser will result in a self-destruct instead of a KO, even if players have been damaged by opponents previously. Characters with invincibility from a Starman will also self-destruct instead of being KO'd.
Super Smash Bros. 4 appears to retain the Brawl system, but with one notable change: in matches of two players, any self-destructs are also credited as KOs for the opponent. For example, if a player has been KO'd once and has one self-destruct, the opponent will be credited with two KOs. This change only affects KO-based statistics; it has no real effect on who wins the match. A cannon resets the KO property as well. For example, if a player got knocked into a cannon on a custom stage and dies, because the cannon shoots him past the blast line, it will count as an SD.
In addition to the colorful KO blast, every fighter makes a loud, distinct vocal sound upon being KO'd, though there are some exceptions who are silent due to lacking voice lines. Throughout the series, these exceptions are Samus (Brawl onwards), Mr. Game and Watch (not in Melee), Olimar, Duck Hunt, Mega Man (excluding "8-bit explosion" sound effects), Pac-Man, Villager and the Mii Fighters.
Almost every fighter in Brawl and Smash 4 possesses two KO sounds, with the second being rarer and often a short phrase or line. In SSB4, there are a handful of characters with only one KO scream, which are Jigglypuff (has two in Japanese), Roy, Mewtwo and Bayonetta. StreetSmash gives Pac-Man a KO sound not heard normally.
List of unique secondary KO lines
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